Document Spring HTTP interface clients

[jamesfredley]

Summary

• document Spring Framework 7 HTTP interface clients in the REST guide
• cover @HttpExchange interfaces, manual synchronous RestClient proxies, grouped bean registration via @ImportHttpServices, and the reactive WebClient alternative
• explicitly frame this as a Spring client API, not a Grails DSL, with error/null limitations

Verification

• :grails-doc:publishGuide -x aggregateGroovydoc
• git diff --check

This is an AI-generated starting point for documenting Spring HTTP interface clients.

[Copilot]

Pull request overview

Adds REST guide documentation for using Spring Framework 7 HTTP interface clients alongside the existing Micronaut HTTP Client guidance, clarifying that this is a Spring client API (not a Grails-specific DSL) and outlining synchronous and reactive options.

Changes:

• Documented @HttpExchange / @GetExchange-based HTTP interface clients.
• Added examples for manual proxy creation via RestClient + HttpServiceProxyFactory.
• Documented grouped bean registration via @ImportHttpServices, plus a note on WebClient for reactive return types and default error/null behavior.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 51.1252%. Comparing base (b980413) to head (1593859).

Additional details and impacted files

@@               Coverage Diff                @@
##             8.0.x     #15996         +/-   ##
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+ Coverage         0   51.1252%   +51.1252%     
- Complexity       0      17601      +17601     
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  Files            0       2041       +2041     
  Lines            0      95493      +95493     
  Branches         0      16587      +16587     
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+ Hits             0      48821      +48821     
- Misses           0      39386      +39386     
- Partials         0       7286       +7286     

see 2041 files with indirect coverage changes

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[testlens-app]

✅ All tests passed ✅

🏷️ Commit: 1593859
▶️ Tests: 25968 executed
⚪️ Checks: 60/60 completed

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