fix(otlp-exporter-base): handle 64KiB limit for browser transports

[YangJonghun]

Which problem is this PR solving?

Browser environments have a 64KiB limit for sendBeacon and fetch with keepalive: true. When the request exceeds this limit:

• sendBeacon returns false
• fetch with keepalive may fail silently in some browsers (Firefox) or throw an error (Chrome)

This causes telemetry data loss for larger batches without any fallback mechanism.

Related #3489

Short description of the changes

1. FailoverTransport: New transport that switches to a failover transport when the primary transport fails
2. FetchTransport: Disable keepalive for body size >= 60KiB (leaving room for headers)
3. SendBeacon delegate: Use FailoverTransport to fall back to fetch when sendBeacon fails
4. Test fixes: Fix sendBeacon stub to return true in existing tests

⚠️ Note

The size-based approach for fetch transport is not a perfect solution since we cannot know the exact final request size (headers, encoding overhead, etc.). However, this change:

• Automatically resolves most issues caused by keepalive being unconditionally set to true in browser environments
• Introduces minimal changes to existing behavior
• Provides a reasonable safety margin (60KiB body limit vs 64KiB total limit)

For sendBeacon, the FailoverTransport provides a more robust solution since sendBeacon explicitly returns false when the request cannot be queued.

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

• Unit tests for FailoverTransport (success, failure, retryable, reject cases)
• Unit tests for FetchTransport keepalive limit (60KiB boundary)
• All existing browser tests pass

npm run test:browser

Checklist:

• Followed the style guidelines of this project
• Unit tests have been added
• Documentation has been updated

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.49%. Comparing base (f15aa7e) to head (2b4f6e0).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6358   +/-   ##
=======================================
  Coverage   95.48%   95.49%           
=======================================
  Files         363      364    +1     
  Lines       11564    11583   +19     
  Branches     2669     2671    +2     
=======================================
+ Hits        11042    11061   +19     
  Misses        522      522           

Files with missing lines	Coverage Δ
...rter-base/src/otlp-browser-http-export-delegate.ts	63.63% <100.00%> (+3.63%)	⬆️
...-exporter-base/src/transport/failover-transport.ts	100.00% <100.00%> (ø)
...tlp-exporter-base/src/transport/fetch-transport.ts	91.42% <100.00%> (+0.51%)	⬆️

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