fix(otlp): don't configure an automatic OTel propagator

[jamescrosswell]

Closes #5299

Summary

The OTLP integration spec states:

The integration MUST NOT set up an automatic propagator. Cross-service trace propagation is handled by the propagateTraceparent setting or by user-configured OTel propagators.

Our OTLP exporter was inconsistent with this: AddSentryOtlpExporter (and UseOtlp) called Sdk.SetDefaultTextMapPropagator(new SentryPropagator()), automatically configuring a global propagator. This brings us in line with the spec and with our other OTLP integrations.

Changes

• Remove the Sdk.SetDefaultTextMapPropagator(...) call from AddSentryOtlpExporter.
• Remove the now-purposeless defaultTextMapPropagator parameter from both AddSentryOtlpExporter and SentryOptionsExtensions.UseOtlp.
• Update XML docs to point users at PropagateTraceparent / their own OTel propagator config.
• Drop the obsolete custom-propagator test; add a regression test asserting AddSentryOtlpExporter leaves Propagators.DefaultTextMapPropagator untouched.

⚠️ Breaking API change. The defaultTextMapPropagator parameter shipped in 6.5.0/6.6.0 and is removed here (source + binary breaking for anyone who passed it). Flagging for a call on whether this should land in a major, or whether you'd prefer to [Obsolete] the parameter first. Note the classic (non-OTLP) Sentry.OpenTelemetry package is intentionally left unchanged — it still requires SentryPropagator.

Users who want cross-service propagation should now either set SentryOptions.PropagateTraceparent or configure OpenTelemetry propagators themselves (e.g. Sdk.SetDefaultTextMapPropagator(...)).

Follow-up

• Docs need a matching update so the OTLP integration page explains propagator configuration — see docs: Add missing OTLP integration docs and fix drift across SDKs sentry-docs#18441 (discussion r3463279950).

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.16%. Comparing base (6690704) to head (87baa96).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5319      +/-   ##
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- Coverage   74.16%   74.16%   -0.01%     
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  Files         508      508              
  Lines       18353    18351       -2     
  Branches     3586     3585       -1     
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- Hits        13612    13610       -2     
  Misses       3869     3869              
  Partials      872      872              

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