Add carrier lookup endpoint

[karlTGA]

Closes #16.

utils::carrier::lookup already resolved operator/country from the compiled-in table, but it was only reachable via /cell and /cells, which need a full cell key. A caller holding just an MCC/MNC had no way in. This wires that lookup to a route: no DB, no new dependency, no new data — 30 lines of source.

API

GET /carrier?mcc=<mcc>&net=<mnc>

net is canonical, mnc accepted as a serde alias — /cell uses net, /cells uses mnc, so this takes both rather than picking a side.

Request	Response
?mcc=262&net=2	200 {"operator":"Vodafone","country":"Germany","countryCode":"DE"}
?mcc=262&mnc=2	200 — alias
?mcc=262&net=999	200 {"operator":null,"country":"Germany","countryCode":"DE"}
?mcc=999&net=1	404 null
?mcc=262	400

A known MCC with an unknown MNC stays 200 with the country populated — that fallback is the behaviour #12 deliberately built, and it is what a UI most wants.

Note the divergence from /cell, called out explicitly in the README: /carrier answers an unknown MCC with 404, while /cell answers a miss with 200 null.

The 404 predicate

carrier == Carrier::default(), which means neither the exact (mcc, net) row nor the MCC fallback row matched. Checked against the table: 3,628 rows, 238 distinct MCCs, 232 have a fallback row, and no row is all-null.

It diverges from a literal "unknown MCC" only for the 6 fallback-less MCCs — 1, 901, 902, 991, 995, 999, test and satellite ranges — with an unmatched MNC, which 404 rather than returning 200 full of nulls. For those, 404 is arguably the better answer. The ceiling and its upgrade path are marked with a ponytail: comment.

Deliberate deviation

The handler skips the query_*/handle_* split documented in CLAUDE.md. That split exists to make DB queries unit-testable against a container connection; this handler touches no database, needs no Config, and is sync. carrier_route() is extracted next to health_route() instead, which is what lets the tests assert real status codes. Noted in CLAUDE.md.

Testing

Five TDD cycles, each red before green, asserting status and body through warp::test::request. Full suite: 81 passing. Clippy: 5 warnings, all pre-existing, none in new code.

Also verified end-to-end against a running server — every row in the table above, plus CORS on the new route and /cell unchanged.

Follow-up, not in this PR

GET /carrier?mcc=505&net=1 returns {"operator":"Telstra","country":"Australia","countryCode":null}. 107 rows carry a country with no code (69 of them Australia) because scripts/update-mcc-mnc.py drops any upstream code that is not exactly 2 chars, and Australia is listed upstream as AU/CX/CC/NF. That is a #12 data-generation bug, not a routing bug, and fixing it means regenerating a 3,600-row CSV — kept separate so both stay reviewable. Documented as a caveat in the README's Carrier Lookup section; a separate issue still needs filing.

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📒 Files selected for processing (4)

• CLAUDE.md
• README.md
• src/handlers/carrier.rs
• src/utils/server.rs

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• CLAUDE.md
• src/handlers/carrier.rs

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📝 Walkthrough

Walkthrough

This change adds a GET /carrier endpoint. It accepts MCC and network values, supports mnc as an alias, returns serialized carrier data, and documents success and not-found responses.

Changes

Carrier lookup

Layer / File(s)	Summary
Carrier response and lookup contract src/handlers/carrier.rs, src/handlers/mod.rs, src/utils/carrier.rs	The handler resolves carriers from compiled data. Carrier serializes with camelCase fields. Unresolved lookups return JSON null with status 404.
Route wiring and endpoint validation src/utils/server.rs	The server parses carrier queries, registers /carrier, and tests successful lookups, the mnc alias, unknown networks, absent MCCs, missing parameters, and MCCs without fallback rows.
Route and response documentation CLAUDE.md, README.md	The documentation describes the route, query parameters, response fields, lookup behavior, status responses, and independent country field degradation.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Merge Risk: 🔵 Low · up to 79200

The new endpoint returns 404 for unmatched MNC values under a small set of known MCCs without fallback data, which may surprise callers that distinguish an unknown MCC from an unknown carrier. This is a bounded behavior risk and is mergeable with explicit owner awareness or follow-up.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely identifies the main change: adding the carrier lookup endpoint.
Description check	✅ Passed	The description explains the new GET /carrier endpoint, supported parameters, response behavior, testing, and scope. It directly relates to the changeset.
Linked Issues check	✅ Passed	The implementation satisfies issue #16 by adding a GET /carrier endpoint that retrieves carrier and country information from MCC/MNC values, including lookup results and fallback behavior.
Out of Scope Changes check	✅ Passed	The changes remain within scope. The handler, route registration, tests, serialization update, and documentation directly support the carrier lookup endpoint. No unrelated data, dependency, or databas…
Docstring Coverage	✅ Passed	Docstring coverage is 90.91% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 11 functions across 4 files. (2 skipped: 2 …

Full details: Out of Scope Changes check

Explanation

The changes remain within scope. The handler, route registration, tests, serialization update, and documentation directly support the carrier lookup endpoint. No unrelated data, dependency, or database changes were added.

Full details: Docstring Coverage

Explanation

Docstring coverage is 90.91% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 11 functions across 4 files. (2 skipped: 2 unsupported.)

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Inline comments:
In `@src/handlers/carrier.rs`:
- Around line 21-25: Update lookup and the carrier handler to distinguish an
unknown MCC from a known MCC whose lookup returns Carrier::default(). Return an
explicit MCC-match indicator or perform a separate MCC membership check, and
return 404 only when no row exists for the MCC; known MCCs including 1, 901,
902, 991, 995, and 999 must return 200 with nulls for unknown MNCs. Update the
mcc=999 route test to verify this behavior.

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• src/handlers/carrier.rs
• src/handlers/mod.rs
• src/utils/carrier.rs
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