fix: negative-cache property/Connect JSON fetch failures

[JakeSCahill]

Problem

Two of the top 404 sources on docs.redpanda.com come from tooltip data fetches that retry a missing file on every page view:

• 19-property-tooltips.js caches successful properties-JSON fetches for 24h in localStorage, but failures are never cached — when the referenced attachment doesn't exist (release tag drifted ahead of the generated JSON), every streaming page view fires a guaranteed 404 (~17.6k/day for redpanda-properties-v26.1.14.json).
• 16-bloblang-interactive.js builds the Connect JSON URL from latest-connect-version and, on failure, walks a hardcoded fallback-version list — up to 6 404s per page view (~6.4k/day for connect-4.102.0.json), with no caching of failures.

Fix

• Property tooltips: store a failed marker in the existing localStorage cache entry (1h TTL vs 24h for successes, so a fix deploy is picked up quickly; still versioned by latest-redpanda-tag). While fresh, resolve to an empty lookup without fetching.
• Bloblang: track per-URL failure timestamps in localStorage (1h TTL); skip URLs that recently returned an error response. Only deterministic HTTP errors are marked — transient network errors still retry.
• Preview mode (localhost / docs-ui.netlify.app) is unaffected: failures there are never marked, and property-tooltip cache reads were already skipped in preview.

This is defense-in-depth for the storm; the root cause (meta tags referencing JSON that was never generated) is fixed at build time by redpanda-data/docs-extensions-and-macros#224.

Testing

• node --check and npx eslint on both files (only pre-existing max-len warning remains)

🤖 Generated with Claude Code

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ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: 22eca7a9-677f-4486-8437-2b626d04339f

📥 Commits

Reviewing files that changed from the base of the PR and between 6273f77 and 8e76e4c.

📒 Files selected for processing (2)

• src/js/16-bloblang-interactive.js
• src/js/19-property-tooltips.js

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📝 Walkthrough

Walkthrough

The changes add localStorage-backed failure caching for Connect JSON and property JSON fetches. Connect requests skip URLs recorded as recently failed and record non-OK responses outside preview mode. Property data cache entries now distinguish failed fetches from successful results, using a one-hour failure TTL instead of the 24-hour success TTL, and failed fetches store version and timestamp metadata.

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

Sequence Diagram(s)

sequenceDiagram
  participant tryFetchConnectJSON
  participant localStorage
  participant ConnectJSONEndpoint
  tryFetchConnectJSON->>localStorage: Read recent URL failures
  localStorage-->>tryFetchConnectJSON: Return failure status
  tryFetchConnectJSON->>ConnectJSONEndpoint: Fetch URL when not recently failed
  ConnectJSONEndpoint-->>tryFetchConnectJSON: Return non-OK response
  tryFetchConnectJSON->>localStorage: Record URL failure

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🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly names the main change: negative-caching failed property and Connect JSON fetches.
Description check	✅ Passed	The description directly matches the changes, problem statement, fix, and testing for cached JSON fetch failures.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/json-fetch-negative-cache

Warning

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🔧 ESLint

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