fix(storage): require gRPC >= 1.85 for DirectPath over Interconnect

[kalragauri]

DirectPathXdsOverInterconnectOption configures the gRPC channel target as google-c2p:///storage-direct.googleapis.com?force-xds. However, the google-c2p resolver only honors the force-xds query parameter starting in gRPC 1.85.0 (grpc/grpc#42980). In gRPC <= 1.84, the resolver accepts the URI but ignores force-xds:

• Off-GCE, the channel skips xDS and resolves storage-direct.googleapis.com via DNS to public CloudPath (storage.googleapis.com).
• On GCE, the channel enters xDS without the force-xds bootstrap context, times out after ~15s waiting for the non-existent EDS resource, and falls back to CloudPath via RLS.

In both cases, LogChannelConfiguration() still reports transport_type=DirectPathInterconnect.

This PR guards DirectPathXdsOverInterconnectOption behind a compile-time gRPC version check (GrpcForceXdsIsSupported(), modeled on GrpcEnableMetricsIsSafe()):

• In DefaultOptionsGrpc(), only set the ?force-xds endpoint when the linked gRPC version is >= 1.85.0. On older gRPC versions, fall back to the standard endpoint defaults (DirectPath on GCE, CloudPath off-GCE) while preserving DirectPathXdsOverInterconnectOption in Options.
• In LogChannelConfiguration(), emit a WARNING identifying the required and actual gRPC versions (grpc::Version()) when DirectPath over Interconnect is requested via the option or an explicit ?force-xds endpoint on gRPC < 1.85.0.
• Document the gRPC >= 1.85.0 requirement on DirectPathXdsOverInterconnectOption.

[gemini-code-assist]

Code Review

This pull request introduces compatibility checks for DirectPath over Interconnect, which requires gRPC 1.85.0 or later to support the 'force-xds' parameter. It adds overloads to DefaultOptionsGrpc and LogChannelConfiguration to handle cases where the gRPC version is older, falling back to standard defaults and logging a warning. Comprehensive unit tests have been added to verify these behaviors. There are no review comments to address, and the changes conform to the repository's style guidelines.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.38%. Comparing base (68190fe) to head (469be99).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff            @@
##             main   #16556    +/-   ##
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  Coverage   92.38%   92.38%            
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  Files        2265     2265            
  Lines      219107   219212   +105     
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+ Hits       202416   202528   +112     
+ Misses      16691    16684     -7     

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