Restore OpenTelemetry stdin schema support for headers and correct sample_rate docs

[Copilot]

The stdin JSON schema still rejected gateway.opentelemetry.headers even though the implementation already parsed and applied it. Separately, the docs regressed to claim JSON stdin support for sampleRate, which is not implemented.

• Schema/runtime alignment

  • Extend the schema fix-up path to inject headers into opentelemetryConfig
  • Keep the change in the existing runtime schema compatibility layer instead of expanding feature surface elsewhere
  • Preserve current behavior for the already-supported stdin → tracing conversion path

• Regression coverage

  • Add schema-level coverage that asserts headers is present in the fixed OpenTelemetry schema
  • Add stdin loading coverage that verifies configs with gateway.opentelemetry.headers now pass validation and populate tracing config

• Documentation correction

  • Update the OpenTelemetry configuration table to mark sample_rate as TOML-only
  • Remove the inaccurate sampleRate (JSON stdin) claim while keeping CLI support documentation intact

• Behavioral example

  {
    "gateway": {
      "port": 8080,
      "domain": "localhost",
      "apiKey": "test-key",
      "opentelemetry": {
        "endpoint": "https://otel-collector.example.com",
        "headers": "X-Test=value"
      }
    }
  }

  This config is now accepted by stdin schema validation and the headers value is propagated into the runtime tracing configuration.

[Copilot]

Pull request overview

This PR restores alignment between the embedded JSON stdin schema and the gateway’s existing OpenTelemetry runtime behavior by allowing gateway.opentelemetry.headers, and it corrects documentation that incorrectly claimed JSON stdin support for sampleRate.

Changes:

• Extend the schema “fix-up” layer to inject headers into opentelemetryConfig so stdin validation no longer rejects a supported field.
• Add tests that (1) assert the schema injection and (2) verify stdin loading accepts gateway.opentelemetry.headers and propagates it into Gateway.Tracing.
• Update OpenTelemetry docs to mark sample_rate as TOML-only (still supported via CLI), removing the inaccurate JSON stdin claim.

Show a summary per file

File	Description
internal/config/validation_schema.go	Injects headers into the fixed Draft-7 schema for opentelemetryConfig to match runtime support.
internal/config/load_from_stdin_coverage_test.go	Adds stdin coverage ensuring gateway.opentelemetry.headers validates and maps to runtime tracing config.
internal/config/fetch_and_fix_schema_test.go	Adds schema-level test coverage for the headers property injection.
docs/CONFIGURATION.md	Corrects docs to indicate sample_rate is TOML-only (not supported via JSON stdin).

Copilot's findings

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• Files reviewed: 4/4 changed files
• Comments generated: 1

[lpcox]

@copilot fix the failing ci check https://github.com/github/gh-aw-mcpg/actions/runs/27022710054/job/79754594049?pr=7042

[Copilot]

@copilot fix the failing ci check https://github.com/github/gh-aw-mcpg/actions/runs/27022710054/job/79754594049?pr=7042

Fixed in 5cf8bb8. I investigated the failing lint job and removed duplicate TestCopyTrimmedStringIntMap* declarations from internal/strutil/util_test.go (the canonical tests remain in internal/strutil/copy_trimmed_string_int_map_test.go), which resolves the go vet redeclaration failure on the PR merge check.