feat: Add request-local caching for repeated SCM lookups [1]

[yk634]

Context

Webhook handling can repeat the same SCM reads multiple times within a single request.
In scm-github, operations such as repository lookup, file fetches, PR info fetches, and author/commit decoration can re-hit the same GitHub endpoints during one webhook flow, which adds avoidable latency and load.

We want to reuse those repeated results within the scope of a single request without changing behavior across requests.

Objective

Introduce request-local caching in scm-github for repeated SCM operations during the same request.

This PR:

• adds an AsyncLocalStorage-backed request cache helper
• caches repeated scm-github lookups such as lookupScmUri, getFile, getOpenedPRs, getRepoInfo, decorateAuthor, decorateCommit, and getPrInfo
• avoids caching failed lookups and unresolved PR mergeability results
• keeps cache keys scoped to the request and to the relevant lookup inputs

References

• feat: Add request cache context for routed SCM calls [2] scm-router#40
• feat: Wrap webhook SCM handling in request cache context [3] screwdriver#3494

License

I confirm that this contribution is made under the terms of the license found in the root directory of this repository's source tree and that I have the authority necessary to make this contribution on behalf of its copyright owner.

[sd-buildbot]

🎉 This PR is included in version 14.6.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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