ci: swap httpbin.org for a locally-hosted version for reliability (doesn't affect local testing)

[talagrand]

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[codecov]

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✅ All modified and coverable lines are covered by tests.

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[Copilot]

Pull request overview

This PR replaces direct calls to https://httpbin.org in tests/real_world.rs with a helper that reads HTTPBIN_URL, and wires CI to run a local go-httpbin instance instead, reducing flakiness from the public service. Local developer runs remain unchanged (falling back to httpbin.org).

Changes:

• Add httpbin(path) helper in tests/real_world.rs and update all httpbin-backed tests/comments to use it.
• New composite action .github/actions/start-httpbin that installs Go, runs go-httpbin on 127.0.0.1:8080, waits for readiness, and exports HTTPBIN_URL; wired into the CI test (stable/full-checks) and coverage jobs with failure-log dumping. Also factors should-run-full-checks into a reusable step output and enables set -euo pipefail / RUST_BACKTRACE=1.
• Add Go module under .github/ci-tools/ pinning go-httpbin v2.23.0 (with Dependabot gomod tracking) and a CHANGELOG entry.

Reviewed changes

Copilot reviewed 6 out of 7 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
tests/real_world.rs	Introduce httpbin() helper; route httpbin tests through HTTPBIN_URL; update doc/log strings.
.github/workflows/ci.yml	Add should-run-full-checks output, start go-httpbin and dump log on failure in test & coverage jobs, set pipefail and RUST_BACKTRACE.
.github/actions/start-httpbin/action.yml	New composite action: install Go, build/start go-httpbin, wait for readiness, export HTTPBIN_URL.
.github/ci-tools/go.mod	New Go module pinning go-httpbin/v2 v2.23.0 via tool directive.
.github/ci-tools/go.sum	Checksums for the pinned go-httpbin module.
.github/dependabot.yml	Track the new gomod directory monthly.
CHANGELOG.md	Add Unreleased "Changed" entry describing the CI swap.

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