chore(shard-manager): provide metadata in constructor

[dkrotx]

What changed?
Refactoring for #7712:
Providing shard executor metadata to the constructor instead of Initalising this separately.
In addition, unifying panic -> logger.Fatal as we do in other cases.

Why?

+Better readability as code is more aligned with other usages of log.Fatal.
Passing metadata to constructor allows to avoid partial structure initialization.

How did you test it?
go test -v ./service/matching/handler

Potential risks

Release notes

Documentation Changes

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Reviewer Validation

PR Description Quality (check these before reviewing code):

• "What changed" provides a clear 1-2 line summary
  • Project Issue is linked
• "Why" explains the full motivation with sufficient context
• Testing is documented:
  • Unit test commands are included (with exact go test invocation)
  • Integration test setup/commands included (if integration tests were run)
  • Canary testing details included (if canary was mentioned)
• Potential risks section is thoughtfully filled out (or legitimately N/A)
• Release notes included if this completes a user-facing feature
• Documentation needs are addressed (or noted if uncertain)

[gitar-bot]

Code Review ✅ Approved

Clean refactoring that moves metadata into the executor constructor and unifies error handling from panic to logger.Fatal. The reorder in GetShardProcess is semantically equivalent. No issues found.

Rules ✅ All requirements met

Repository Rules

✅ PR Description Quality Standards: PR description meets all quality standards. [What changed?] has clear summary with issue #7712 linked. [Why?] explains technical rationale including 'Passing metadata to constructor allows to avoid partial structure initialization' and alignment with other log.Fatal usages. [How did you test it?] provides exact command: go test -v ./service/matching/handler.

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