feat(bigtable): support DirectPath in Cloud Bigtable C++ client

[elinorli]

Extract DirectPath checking logic into a reusable IsDirectPath() function and update option defaults and metadata flags:

• Extract environment variable checking logic to bigtable::internal::IsDirectPath()
• Check both GOOGLE_CLOUD_ENABLE_DIRECT_PATH and CBT_ENABLE_DIRECTPATH
• Set c2p:///bigtable.googleapis.com as the endpoint when DirectPath is enabled
• Do not limit channel counts to 1 when DirectPath is enabled
• Update features metadata in the stub factory to request DirectPath when enabled
• Add test coverage for new environment variable checks and feature flags

[gemini-code-assist]

Code Review

This pull request introduces support for the CBT_ENABLE_DIRECTPATH environment variable, updates the Direct Path endpoint to c2p:///bigtable.googleapis.com, and removes the single-channel restriction for Direct Path. It also adds logic to include specific feature flags in the bigtable-features metadata when Direct Path is active. A critical memory leak was identified in FeaturesMetadata() because the static keyword was removed from a heap-allocated variable; this should be corrected by restoring the static qualifier and moving the environment check inside the initialization lambda.

[codecov]

Codecov Report

❌ Patch coverage is 98.63014% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.70%. Comparing base (d48786e) to head (4984937).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...ud/bigtable/internal/bigtable_stub_factory_test.cc	97.29%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16088      +/-   ##
==========================================
- Coverage   92.70%   92.70%   -0.01%     
==========================================
  Files        2353     2353              
  Lines      218231   218352     +121     
==========================================
+ Hits       202313   202421     +108     
- Misses      15918    15931      +13     

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