fix(cluster): improve streaming dispatch logging and observability

[vroldanbet]

Summary

• Enrich log messages with request context: propagate log.Ctx(ctx) throughout all dispatch log calls (trace/warn/debug) so request-scoped fields (e.g. requestID) are included in every log line
• Increase log timestamp resolution to nanoseconds: set zerolog.TimeFieldFormat = time.RFC3339Nano globally and add WithTimestampFormat(time.RFC3339Nano) to all gRPC log interceptors, helping log aggregators order events correctly
• Add deadline to streaming dispatcher contexts: embed the context deadline in the logger attached to primary and secondary streaming dispatch contexts for easier debugging
• Fix non-empty placeholder for dispatcher results: replace empty string "" with $$no_dispatcher_results constant as the initial value of returnedResultsDispatcherName, making the no-results vs. empty-name distinction explicit and preventing a silent bug
• Code quality: rename withTimeout → ctxWithTimeout to avoid var shadowing, pass context explicitly to addResultTime, pass handler context explicitly to runHandler, add guard against empty dispatcher names

Test plan

• Run existing cluster dispatch unit tests: go test ./internal/dispatch/remote/...
• Verify log output includes requestID and nanosecond timestamps for dispatch-related log lines
• Confirm streaming dispatch still works end-to-end with secondary dispatchers configured

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 75.34247% with 18 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.52%. Comparing base (05f1566) to head (16c885e).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/dispatch/remote/cluster.go	74.63%	15 Missing and 2 partials ⚠️
cmd/spicedb/main.go	0.00%	1 Missing ⚠️

❌ Your project check has failed because the head coverage (74.52%) is below the target coverage (75.00%). You can increase the head coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2915      +/-   ##
==========================================
+ Coverage   74.51%   74.52%   +0.02%     
==========================================
  Files         484      484              
  Lines       59201    59242      +41     
==========================================
+ Hits        44109    44146      +37     
- Misses      11995    11999       +4     
  Partials     3097     3097              

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[josephschorr]

LGTM