Strengthen structured logging for composition/scatter-gather fallback paths

[Copilot]

This PR tightens logging around degraded execution paths and aligns warning logs with Microsoft.Extensions.Logging structured conventions. Scope is intentionally narrow: improve observability without changing request behavior.

• Structured warning templates (response serialization config)

  • Replaced interpolated warning text with a message template + named property in ResponseSerializationOptions.ValidateConfiguration(...).
  • Preserves current warning semantics while making logs filterable/aggregatable.

• Explicit warning for ignored downstream scatter/gather failures

  • HttpGatherer.Gather(...) now logs Warning when IgnoreDownstreamRequestErrors swallows HttpRequestException.
  • Emits structured fields:
    • GathererKey
    • DestinationUrl
  • Exception is attached to preserve stack/context in sinks.

• Focused diagnostics assertions

  • Updated logging tests to assert template (OriginalFormat) and structured properties rather than only rendered message text.
  • Improved assertion clarity for property extraction.

• Scope correction from review feedback

  • Reverted unrelated nullability/transform changes in HttpGatherer so the PR remains logging-only.

_logger?.LogWarning(
    ex,
    "Ignoring downstream request error for gatherer {GathererKey} at {DestinationUrl}.",
    Key,
    destination);

PoA

• PR description
• Apply labels as appropriate
• tests
• documentation

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