Implement strict trace continuation (org_id validation)

[giortzisg]

Summary

Implement the strictTraceContinuation option as specified in the SDK spec.

This prevents unintended trace mixing across different Sentry organizations by validating the org_id in incoming baggage headers.

Requirements

1. Parse org_id from DSN

Extract the organization ID from the DSN host (e.g., o1234 from o1234.ingest.sentry.io → "1234").

2. Add org_id configuration option

Allow explicit override of the DSN-derived org_id (needed for self-hosted / Relay setups).

3. Propagate org_id in baggage

Add sentry-org_id to the Dynamic Sampling Context (DSC) / baggage when creating head-of-trace.

4. Implement trace continuation validation

Add a strict_trace_continuation option (default: false) that validates incoming sentry-org_id against the SDK's org_id before continuing a trace. See the spec for the full decision matrix.

[linear]

RUST-166