Strengthen TOML parse-error guarantees and clarify tracing key precedence

[Copilot]

This update tightens BurntSushi/toml integration in internal/config by ensuring structured parse errors remain unwrap-able and by making dual tracing-key behavior explicit. It also documents an intentional TOML key naming exception in guard policy config.

• Parse error contract hardening

  • TestLoadFromFile_InvalidTOML now asserts the returned error unwraps to toml.ParseError and carries non-zero line/column position metadata.
  • This protects against future wrapping regressions that would drop structured parse context.

• Tracing key precedence visibility

  • LoadFromFile now emits a warning when both [gateway.tracing] and [gateway.opentelemetry] are present.
  • Existing behavior is preserved: opentelemetry continues to override legacy tracing.

• Precedence behavior coverage

  • Added TestLoadFromFile_BothTracingAndOpenTelemetry_OpenTelemetryTakesPrecedence to verify merged result uses opentelemetry values and clears the temporary alias field.

• Guard policy key documentation

  • Added a focused comment on GuardPolicy noting hyphenated TOML keys (e.g. allow-only) are intentional to match policy schema field names.

if cfg.Gateway.Opentelemetry != nil {
    if cfg.Gateway.Tracing != nil {
        logConfig.Print("Warning: both [gateway.tracing] and [gateway.opentelemetry] are set; [gateway.opentelemetry] takes precedence")
    }
    cfg.Gateway.Tracing = cfg.Gateway.Opentelemetry
    cfg.Gateway.Opentelemetry = nil
}

[Copilot]

Pull request overview

Tightens TOML integration in internal/config by asserting that parse errors expose structured toml.ParseError data, makes the precedence between [gateway.tracing] and [gateway.opentelemetry] explicit with a warning log and test, and documents intentional hyphenated TOML key names on GuardPolicy.

Changes:

• Emit a warning when both tracing and opentelemetry gateway sections are set; opentelemetry still wins.
• Strengthen invalid-TOML test to verify errors.As(toml.ParseError) with non-zero line/column.
• Add precedence test covering combined tracing + opentelemetry config and document hyphenated keys on GuardPolicy.

Show a summary per file

File	Description
internal/config/config_core.go	Logs warning when both tracing and opentelemetry are configured.
internal/config/config_core_test.go	Adds parse-error structural assertions and a precedence test.
internal/config/guard_policy.go	Comment clarifying intentional hyphenated TOML keys.

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• Files reviewed: 3/3 changed files
• Comments generated: 0