feat(rust): add mergify-core::http with retry + typed errors (Phase 1.2b)

[jd]

Adds the HTTP client every ported command uses. Wraps reqwest
with the invariants the design requires:

• Bearer-token auth injected when the token is non-empty; skipped
  otherwise so GitHub's anonymous public-repo reads still work.
• Tenacity-style retry on 5xx and transient network errors: 3
  attempts, exponential backoff starting at 1s. 4xx responses are
  never retried — those are caller errors and retrying would mask
  bugs.
• Typed error mapping: HTTP failures become either
  CliError::GitHubApi or CliError::MergifyApi based on the
  ApiFlavor passed at construction, so exit_code() routes
  to GITHUB_API_ERROR (5) or MERGIFY_API_ERROR (6)
  automatically.
• Per-request timeout of 30s.
• RetryPolicy is public so tests can skip the wall-clock
  backoff with initial_backoff = 0.

Also extends CliError with the two new API variants and
updates the exit_code() mapping + unit tests.

Command crates MUST NOT import reqwest directly — the only
HTTP surface is HttpClient::get and HttpClient::post.

6 new tests cover: happy-path GET, empty-token behavior, POST
with JSON body, retry on 5xx then success, exhausted retries
mapping to MergifyApi, 4xx not retried and mapping to
GitHubApi. All use wiremock for hermetic local HTTP.

Next sub-phase (1.2c / 1.2d) adds colored/spinner affordances and
the GitOps trait only when the first ported command needs
them; this PR is the minimum config validate requires.

Co-Authored-By: Claude Opus 4.7 (1M context) noreply@anthropic.com

Depends-On: #1280

[jd]

This pull request is part of a Mergify stack:

#	Pull Request	Link
1	feat(rust): scaffold Cargo workspace and Rust CI	#1263
2	feat(rust): add mergify-py-shim with embedded Python fallback	#1272
3	feat(rust): add mergify-core foundation types (Phase 1.2a)	#1280
4	feat(rust): add mergify-core::http with retry + typed errors (Phase 1.2b)	#1281	👈
5	feat(rust): port config validate to native Rust (Phase 1.3)	#1282

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[jd]

Revision history

#	Type	Changes	Date
1	initial	61966dc	2026-04-21 13:13 UTC
2	rebase	61966dc → 0e72eb6	2026-04-21 13:13 UTC