resolver: add ValidateTargetURI for target URI validation

[hugehoo]

• updates Add an internal API to validate a target URI #8747

Changes

• add validation utility for target(grpc server) URI.
• considered:
  • it parses as an RFC 3986 URI via net/url.Parse, and
  • its scheme has a resolver builder registered in the global resolver registry (resolver.Get).

Scope

• PR intentionally contains only the utility and its unit tests.
• Adoption in xDS bootstrap and other configuration-parsing paths will follow in separate PRs.

Testing

• Table-driven tests cover:
  • Registered scheme, both authority form (scheme:///endpoint) and opaque form (scheme:endpoint)
  • Uppercase input for a registered scheme. url.Parse canonicalizes the scheme to lowercase per RFC 3986 §3.1, so the case difference does not surface at the registry lookup; the test pins this behavior.
  • Empty target.
  • host:port without a scheme, which is parsed as an unregistered scheme and so the common misconfiguration surfaces cleanly.
  • Unregistered scheme.
  • Unparseable input.

---

RELEASE NOTES: none

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.26%. Comparing base (7d87045) to head (83f750a).
⚠️ Report is 39 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9247      +/-   ##
==========================================
- Coverage   83.27%   83.26%   -0.01%     
==========================================
  Files         421      422       +1     
  Lines       34084    34096      +12     
==========================================
+ Hits        28382    28390       +8     
- Misses       4273     4274       +1     
- Partials     1429     1432       +3     

Files with missing lines	Coverage Δ
balancer/rls/config.go	86.36% <100.00%> (+0.75%)	⬆️
internal/resolver/target.go	100.00% <100.00%> (ø)
internal/xds/bootstrap/bootstrap.go	64.15% <100.00%> (+0.21%)	⬆️

... and 24 files with indirect coverage changes

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[easwars]

@hugehoo : Could you please also update the callsites that you plan to change to use this helper. I think we got stuck the last time around when we got to that part. So, I don't want to add a utility without changing the callsites and knowing for sure that we have exactly what we need. Thanks for understanding.

[hugehoo]

@easwars thanks for review!

I've updated the callsites to use this helper, along with their tests:

• balancer/rls/config.go: replaced the inline lookup_service validation with the helper.
• internal/xds/bootstrap: ServerConfig.UnmarshalJSON now validates server_uri.

The helper now applies the default-scheme fallback for targets that don't parse as authority-form URIs, while still rejecting authority-form URIs (scheme://...) with an unregistered scheme, so scheme typos in configuration surface as errors.
This accepts existing host:port-style bootstrap configs and preserves RLS's existing rejection of targets like badScheme:///target.

[hugehoo]

updated scheme to passthrough from ipv4 as server_uri validation added.