Add observable provider attempt metadata

[clifton]

Summary

• preserve exact HTTP status codes and recognized provider request IDs on every extraction attempt and provider error
• add opt-in bounded response-body capture behind a mandatory caller-supplied sanitizer
• emit one content-free OpenTelemetry GenAI semantic-convention span per non-streaming extraction or generation attempt through the existing tracing facade
• classify malformed successful provider envelopes as typed UnexpectedResponse errors while retaining their HTTP diagnostics

API notes

• AttemptRecord::response carries the uniform ResponseMetadata sidecar on both success and failure
• RStructorError::{status_code, request_id, response_metadata} expose terminal diagnostics without matching provider-specific variants
• ResponseBodyCapture never stores a body unless the caller explicitly opts in and supplies the sanitizer
• applications keep control of their tracing subscriber and OpenTelemetry SDK/exporter

RStructorError::ApiError has a new response metadata field. Exact downstream struct-pattern matches should add ..; normal matches and accessor-based code are unchanged.

The GenAI semantic conventions are still marked development upstream, so the instrumentation exports GEN_AI_SEMCONV_STABILITY and isolates field construction in one module for future convention updates.

Verification

• cargo fmt --all -- --check
• cargo clippy --all-targets -- -D warnings
• cargo clippy --all-targets --all-features --target-dir tmp/pr3-target -- -D warnings
• cargo check --no-default-features --features derive,mock --target-dir tmp/pr3-min-target
• deterministic all-feature integration matrix (credentialed live-provider modules skipped)
• commit-hook unit suite: 219 passed
• all-feature library suite: 306 passed
• HTTP mock suite: 44 passed