fix(bigtable): use google-c2p:// as the Bigtable DirectPath endpoint

[elinorli]

Fix to use the correct Bigtable DirectPath endpoint, similar to storage.

[gemini-code-assist]

Code Review

This pull request updates the Bigtable Direct Path endpoint from "c2p:///bigtable.googleapis.com" to "google-c2p:///bigtable.googleapis.com" in the default options and corresponding test cases. I have no feedback to provide.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.70%. Comparing base (55df15b) to head (5ff724e).

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #16092   +/-   ##
=======================================
  Coverage   92.70%   92.70%           
=======================================
  Files        2353     2353           
  Lines      218352   218354    +2     
=======================================
+ Hits       202418   202423    +5     
+ Misses      15934    15931    -3     

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