feat(cadence-matching): Add drain observer for SD executor client

[gazi-yestemirova]

What changed?
This PR adds DrainObserver field to resource.Params to inject a DrainSignalObserver into the matching service at initialization.
We wire the drain observer through matching.Service -> handler.NewEngine -> setupExecutor -> executorclient.Params, enabling the executor client's heartbeat loop to react to infrastructure drain/undrain signals.

Why?
The shard-distributor executor client already supports drain-aware behavior via the DrainObserver field on executorclient.Params. However, there was no way to wire a DrainSignalObserver into the matching service's executor client because resource.Params didn't have it.
PR follows the same pattern used by ShardDistributorMatchingConfig initialization.

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

Potential risks
Since drainObserver field is optional there wouldn't be any changes in cadence-matching's behaviour.

Release notes
N/A

Documentation Changes
N/A

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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 parameter threading for drain observer through the matching service initialization chain. Follows the established pattern, is nil-safe, and tests are updated appropriately.

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