emit per-call jsRpcCall spans and propagate stub origin for trace continuity

[apeacock1991]

Imported from #6750. Dupe of #6966 but named correctly (branch matching internal).

Summary

Add per-call tracing for JavaScript RPC.

Changes

• Emit jsRpcCall spans on both caller and callee.
• Tag spans with the RPC method, operation, and target kind.
• Parent the callee onset to the call that opened the session.
• Add jsrpc.caller_span_id to correlate each callee dispatch with its caller.
• Preserve tracing relationships for pipelined calls, returned stubs/promises, and callbacks.
• Avoid retaining span state on untraced paths.

Testing

• Added streaming-tail tests for pipelined RPC calls.
• Added coverage for callback argument tracing.
• Updated existing tail-worker tracing expectations.

Screenshots

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[codspeed-hq]

Merging this PR will degrade performance by 1.97%

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

⚡ 1 improved benchmark
❌ 1 regressed benchmark
✅ 70 untouched benchmarks
⏩ 129 skipped benchmarks¹

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
❌	JsString_Utf8Length_Utf16_Invalid_NonFlat_256	9.4 µs	10.7 µs	-12.74%
⚡	bm_Promise_Fib10	20.3 µs	18.5 µs	+10.12%

Tip

Investigate this regression by commenting @codspeedbot fix this regression on this PR, or directly use the CodSpeed MCP with your agent.

---

_{Comparing apeacock/jsrpcTracing-gitlab (fca229f) with main (c15dd17)}

Footnotes

1. 129 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

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