fix(fetch): preserve explicit trace context during propagation

[mandar1045]

Tl;dr

Fixes supabase/supabase#50006

Preserves caller-supplied traceparent and tracestate headers during function-to-function invocations.

What changed

• In vendor/deno_fetch/26_fetch.js, skip injecting the trace context propagator if the request already contains an explicit traceparent header.
• Keeps traceparent and tracestate paired together from the caller, preventing the runtime from appending a second trace context or attaching mismatched runtime tracestate.
• Automatic trace propagation continues to work when no explicit header is provided.

Validation

• cargo test -p base --test integration_tests test_fetch_preserves_explicit_trace_context (passed)
• Added 3 regression tests checking exact traceparent matching, comma prevention, and tracestate pairing.