feat(observability): query gateway core + dashboard shell

[Ma77Ball]

What changes were proposed in this PR?

Introduces the tenant-scoped read path the dashboard queries, plus the Angular shell that hosts the per-signal panels. This PR warrants the closest review: it enforces tenancy and rate limiting and is the first user-visible surface.
Backend:

• BackendClient: HTTP client to the telemetry backends.
• ScopeResolver: derives the caller's tenant scope and constrains every query to it.
• RateLimiter, AuditLogger, GatewayContext: per-request rate limiting, audit logging, and shared request context.
• dtos.scala: typed request objects with validators (time window, page size, free text, service name).
• ObservabilityResources with the /observability/health endpoint, registered in TexeraWebApplication.
• RequestContextMdcFilter and UserContextMdcFilter: inject request and user context into the logging MDC.
• ObservabilityGatewayConfig and its configuration file.
  Frontend:
• Observability dashboard page, route, and navigation entry, plus observability.service, observability.types, and the traces-pivot.service.
• Health gating: each tab is guarded by the per-signal reachability check; an unreachable signal renders an explicit state rather than a broken panel. Signal panels follow in PR5 through PR8.

Any related issues, documentation, or discussions?

Closes: #5370
Part of #4070. Stacked on #5377.

How was this PR tested?

• Backend specs for the gateway core, DTO validation, scope resolver, rate limiter, and MDC filters; sbt scalafmtCheckAll passes.
• Frontend component and service specs; prettier-eslint and eslint pass.
• Compile and the full test suites run in this PR's CI.

Was this PR authored or co-authored using generative AI tooling?

Co-authored with Claude Opus 4.8 in compliance with ASF

[codecov-commenter]

Codecov Report

❌ Patch coverage is 65.50000% with 483 lines in your changes missing coverage. Please review.
✅ Project coverage is 55.17%. Comparing base (6d31f46) to head (71ac9ce).
⚠️ Report is 52 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
...apache/texera/web/observability/gateway/dtos.scala	41.98%	100 Missing and 5 partials ⚠️
...ala/org/apache/texera/observability/OtelInit.scala	63.30%	43 Missing and 8 partials ⚠️
...texera/web/observability/gateway/AuditLogger.scala	0.00%	41 Missing ⚠️
...observability/gateway/ObservabilityResources.scala	0.00%	38 Missing ⚠️
...xera/web/observability/gateway/ScopeResolver.scala	19.44%	27 Missing and 2 partials ⚠️
...kspace/service/notebook-migration/migration-llm.ts	79.61%	14 Missing and 7 partials ⚠️
...era/web/observability/gateway/GatewayContext.scala	0.00%	19 Missing ⚠️
...a/org/apache/texera/web/TexeraWebApplication.scala	0.00%	15 Missing ⚠️
...org/apache/texera/observability/TexeraTracer.scala	0.00%	14 Missing ⚠️
...wer-button/computing-unit-selection.component.html	0.00%	14 Missing ⚠️
... and 27 more

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #5378      +/-   ##
============================================
+ Coverage     53.86%   55.17%   +1.31%     
- Complexity     2756     3017     +261     
============================================
  Files          1099     1140      +41     
  Lines         42541    44114    +1573     
  Branches       4577     4819     +242     
============================================
+ Hits          22916    24342    +1426     
- Misses        18290    18326      +36     
- Partials       1335     1446     +111     

Flag	Coverage Δ		*Carryforward flag
access-control-service	70.14% <100.00%> (-0.30%)	⬇️
agent-service	34.36% <ø> (ø)		Carriedforward from f6bf45d
amber	57.43% <57.92%> (+2.23%)	⬆️
computing-unit-managing-service	0.00% <0.00%> (-1.66%)	⬇️
config-service	50.76% <40.00%> (-5.95%)	⬇️
file-service	58.88% <68.96%> (+1.81%)	⬆️
frontend	48.96% <83.65%> (+0.90%)	⬆️
notebook-migration-service	78.57% <ø> (?)
pyamber	90.15% <100.00%> (+0.02%)	⬆️	Carriedforward from f6bf45d
python	90.76% <ø> (-0.04%)	⬇️	Carriedforward from f6bf45d
workflow-compiling-service	54.74% <ø> (-3.96%)	⬇️

*This pull request uses carry forward flags. Click here to find out more.

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[github-actions]

Automated Reviewer Suggestions

Based on the git blame history of the changed files, we recommend the following reviewers:

• Contributors with relevant context: @bobbai00, @aicam, @Yicong-Huang
  You can notify them by mentioning @bobbai00, @aicam, @Yicong-Huang in a comment.