Type: Feature client: Add WithTargetCheck DialOption for early target validation

[sithu-go]

Fixes #8739

Motivation

When migrating from the deprecated grpc.Dial with WithBlock() and WithTimeout() to grpc.NewClient, there is no built-in way to validate target reachability during client initialization without blocking behavior. This PR introduces WithTargetCheckas an alternative that validates DNS resolution and TCP reachability without the complexity and anti-patterns associated with WithBlock().

Use Case
This option is valuable for scenarios where:

• Services need to "fail fast" during startup if critical backend dependencies are unreachable
• Early validation prevents cascading failures in orchestrated systems
• Development and testing environments benefit from immediate feedback

Unlike WithBlock(), WithTargetCheck performs a single validation check during NewClient() and does not interfere with the connection lifecycle or background reconnection logic.

Implementation

This PR adds:

1. New DialOption: WithTargetCheck(timeout time.Duration) - performs DNS resolution and TCP dial validation for "dns" and "passthrough" schemes
2. Validation logic: Added validateTarget method to ClientConn that attempts a TCP connection within the specified timeout
3. Tests: unit tests covering success and failure cases

The implementation:

• Only validates during NewClient() initialization
• Returns an error immediately if the target cannot be reached within the timeout
• Supports "dns" and "passthrough" schemes (other schemes are skipped)
• Does not block ongoing connection attempts or interfere with reconnection behavior

RELEASE NOTES:

client: Added WithTargetCheck DialOption to validate target DNS resolution and TCP reachability during client initialization, providing a lightweight alternative to deprecated WithBlock() for fail-fast scenarios.

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: sithu-go / name: Strange (441c2a7, e5eb503)

[codecov]

Codecov Report

❌ Patch coverage is 94.11765% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 82.61%. Comparing base (4c27cc8) to head (441c2a7).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
clientconn.go	92.30%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8743      +/-   ##
==========================================
- Coverage   83.28%   82.61%   -0.68%     
==========================================
  Files         418      418              
  Lines       32367    32399      +32     
==========================================
- Hits        26958    26765     -193     
- Misses       4034     4055      +21     
- Partials     1375     1579     +204     

Files with missing lines	Coverage Δ
dialoptions.go	68.92% <100.00%> (-20.15%)	⬇️
clientconn.go	71.27% <92.30%> (-18.87%)	⬇️

... and 18 files with indirect coverage changes

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