fix(github): align codebase to use a consistent github uri

[albertywu]

Summary

Aligns proto documentation and internal implementation to use a consistent canonical github uri shape:

github://uber/repo/pull/123/<sha>

Before

ParseChangeID silently misparsed input URLs because it wasn't handling /pull/ in the URI:

For example:
github://uber/repo/pull/123/<sha> would parse to Org="uber/repo" Repo="pull" instead of erroring.

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After

• github://uber/repo/pull/123/<sha> parses correctly to Org="uber" Repo="repo"
• parser ensures that <sha> is canonical 40-char lowercase hex and returns error if not. Parsing happens at edge of application such that downstream internals can be assured that the <sha> is always in standard format.

Test Plan

✅ make test

[Copilot]

Pull request overview

Tightens entity/github.ParseChangeID to require the canonical /pull/ literal segment and a full 40-char lowercase hex head commit SHA, matching GitHub's headRefOid. Previously, the missing-/pull/ form silently misparsed (e.g. Repo="pull") and abbreviated/non-hex SHAs would only fail later in the staleness check with a confusing message. All fixtures across entity, gateway, orchestrator, extension, and integration/e2e suites are updated to the canonical form with realistic 40-char SHAs; the proto comment and generated .pb.go are also updated.

Changes:

• Parser now requires a literal pull segment third-to-last and a strict 40-char lowercase hex SHA; String() re-emits the canonical /pull/ form.
• Parser tests cover all new rejection cases (missing/wrong literal, abbreviated/uppercase/non-hex/too-long SHA) and round-tripping for nested-org paths.
• Proto, entity, and downstream doc comments updated to document <scheme>, /pull/ template, full hex SHA requirement, and supported schemes (github, ghe, ghes).

Reviewed changes

Copilot reviewed 22 out of 23 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
entity/github/change_id.go	Adds pullSegment/shaLength constants, enforces literal pull segment and isFullHexSHA, updates String() and format docs.
entity/github/change_id_test.go	Adds shared SHA constants and negative cases for missing/wrong literal segment and SHA validation; updates round-trip cases.
entity/request.go, entity/change_record.go, extension/changeprovider/change_provider.go	Doc-only updates to URI template and example.
entity/request_test.go, entity/land_request_test.go	Fixture URIs updated to canonical /pull/ form with 40-char hex SHAs.
gateway/proto/gateway.proto, gateway/protopb/gateway.pb.go	Proto comment expanded to document template, schemes, and SHA constraints; generated Go mirrors comment.
gateway/controller/land_test.go	Test URIs updated to canonical form.
orchestrator/controller/{start,validate,batch,score,merge}/*_test.go	Fixture URIs updated to canonical form with 40-char SHAs.
extension/mergechecker/github/checker_test.go	Adds shared SHA constants; updates URIs and HeadRefOid values; updates expected error message.
extension/mergechecker/multi_test.go	Updates routing test URIs; unknown-scheme case keeps non-canonical URI since routing fails before parsing.
extension/changeprovider/github/provider_test.go	Adds shared SHA constants; updates URIs/HeadRefOid consistently across cases.
extension/pusher/git/git_pusher_test.go	uri() helper now emits /pull/1/<sha>.
test/integration/gateway/suite_test.go, test/e2e/suite_test.go	Integration/E2E land-request fixtures updated to canonical form.
test/integration/extension/storage/suite.go, test/integration/extension/changestore/mysql/changestore_test.go	Storage/change-store fixture URIs updated to canonical form with 40-char SHAs.

Files not reviewed (1)

• gateway/protopb/gateway.pb.go: Language not supported

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