refactor(common): decouple call_context.h from opentelemetry_context.h

[colinmoy]

google/cloud/stream_range.h includes google/cloud/internal/call_context.h, which previously included opentelemetry_context.h and transitively pulled <opentelemetry/...> headers into every translation unit using StreamRange<T>. This breaks builds that use Clang header modules and strict layering checks when OpenTelemetry headers are not modularized.

Forward-declaring opentelemetry::context::Context in call_context.h and moving CallContext/ScopedCallContext out-of-line into opentelemetry_context.cc avoids leaking OpenTelemetry headers through call_context.h.

[gemini-code-assist]

Code Review

This pull request moves the implementation of CallContext and ScopedCallContext from the header call_context.h to opentelemetry_context.cc to reduce header dependencies. The review feedback highlights a critical ABI and ODR violation hazard introduced by forward-declaring OpenTelemetry types with conditional preprocessor checks in a public header. To resolve this, the reviewer recommends using std::any for type-erasing OTelContext in the header, along with corresponding updates using std::any_cast and including <any> in the source file.

[codecov]

Codecov Report

❌ Patch coverage is 91.66667% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.32%. Comparing base (d932dd6) to head (ae2ad1c).
⚠️ Report is 20 commits behind head on main.

Files with missing lines	Patch %	Lines
google/cloud/internal/opentelemetry_context.cc	91.66%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16557      +/-   ##
==========================================
- Coverage   92.35%   92.32%   -0.03%     
==========================================
  Files        2262     2261       -1     
  Lines      217027   217045      +18     
==========================================
- Hits       200425   200390      -35     
- Misses      16602    16655      +53     

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