Add insta snapshot tests for installation-token error messages, log_timing output, and Debug redaction

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Background

PR #99 adds InstallationAccessToken with TokenAcquisitionFailed error messages,
log_timing structured output, and a manual Debug impl that redacts the token field.
These are currently verified with brittle substring assertions.

Work required

• Add insta to dev-dependencies in Cargo.toml if not already present.
• Add snapshot tests in src/github/installation_token_tests.rs (or a dedicated snapshot
  test file) covering:
  • Each GitHubError::TokenAcquisitionFailed message string produced by
    InstallationAccessToken::new, from_metadata, and classify_token_error.
  • The Debug formatting of InstallationAccessToken (redaction check).
  • The structured fields emitted by log_timing (use tracing-subscriber/tracing-test
    or insta JSON snapshots of captured log events).
• Run cargo insta review to accept baseline snapshots.

References

• PR Acquire installation tokens with an expiry buffer (3.2.1) #99: Acquire installation tokens with an expiry buffer (3.2.1) #99
• Raised by @leynos

[coderabbitai]

🔗 Related PRs

leynos/mdtablefix#276 - Cli matrix testing [merged]
leynos/tei-rapporteur#79 - Use named validation BDD scenarios (#34) [closed]
leynos/dear-diary#26 - Add cargo-binstall release support [merged]
leynos/mdtablefix#303 - Add observability for inline wrapping token classification (#296) [merged]
#107 - Mark 2.6.2 runtime ACP method denylist as done on the roadmap (2.6.2) [merged]

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