docs(trace): clarify that startActiveSpan requires manual span.end()

[raphaeltorquat0]

Which problem is this PR solving?

The JSDoc for startActiveSpan does not make it clear that the callback function is responsible for calling span.end(). Users coming from other OpenTelemetry implementations (like Go) where spans are automatically ended may silently lose spans when using the JavaScript SDK.

This PR adds explicit documentation warning that:

1. The callback is responsible for calling span.end()
2. The span is NOT automatically ended when the callback returns
3. If span.end() is not called, the span will never be exported

Fixes #5905

Short description of the changes

Added an "Important" note to the JSDoc for startActiveSpan in both:

• api/src/trace/tracer.ts (the interface)
• packages/opentelemetry-sdk-trace-base/src/Tracer.ts (the implementation)

The note explicitly states that span.end() must be called by the callback function, otherwise the span is silently lost.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Note: parts of this implementation were developed with AI assistance.

---

CLA Status: I have signed the Contributor License Agreement for this project.

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: raphaeltorquat0 / name: Raphael Torquato (f73f7b0)

[raphael-theriault-swi]

LGTM

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.70%. Comparing base (4c0f3f1) to head (f73f7b0).
⚠️ Report is 58 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6633   +/-   ##
=======================================
  Coverage   95.70%   95.70%           
=======================================
  Files         375      375           
  Lines       12921    12921           
  Branches     3090     3090           
=======================================
  Hits        12366    12366           
  Misses        555      555           

Files with missing lines	Coverage Δ
...ackages/opentelemetry-sdk-trace-base/src/Tracer.ts	98.71% <ø> (ø)

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[otelbot]

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