test(otlp-common): expand test coverage for OTLP common exporter

[arjun-rajappa]

Closes #1667
Expands test coverage for the OTLP common exporter and adds span flags support per OTLP specification.

Changes
Added build_span_flags method to set remote parent context flags on spans and links
Comprehensive test coverage for all private methods (as_otlp_span, as_otlp_status_code, as_otlp_span_kind, as_otlp_key_value, as_otlp_any_value)
Integration tests for complex spans with multiple resources and scopes
Enhanced error handling tests for encoding issues
Testing
All tests pass. Coverage expanded from basic functionality to comprehensive edge cases.

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[kaylareopelle]

Hi @arjun-rajappa, thanks for your patience with my review. Could you take a look at the failing tests?

[thompson-tomo]

@kaylareopelle i have split out the new test cases to a seperate PR #2159 to enable this to be more focussed.

@arjun-rajappa it would be great if you could avoid mixing in format changes as it just ends up making it more time consuming to review.

[arjun-rajappa]

Hi @arjun-rajappa, thanks for your patience with my review. Could you take a look at the failing tests?

I believe these failures are addressed already in #2074. As these are related to otlp-http. Once the other PR is merged in these should start passing.

[thompson-tomo]

I don't think that is the case as our main is passing so a change here is resulting in the failure.

Note common is a dependency of http hence why it is possible a change to common could lead to failures in http.

[arjun-rajappa]

These changes are closely related, as otlp-http relies on methods defined in otlp-common. As a result, changes to otlp-common can impact the tests in otlp-http. If needed, I can add the changes here or rebase this branch entirely on top of #2074.