[JAVA-SPRING;KOTLIN-SPRING]: honor useTags=false for HTTP interface libraries

[wing328]

based on #24862 with resolved merge conflicts

• SpringCodegen: include spring-http-interface in supportLibraryUseTags() so useTags=false groups operations by first path segment instead of tag.
• KotlinSpringServerCodegen: include spring-declarative-http-interface in the addOperationToGroup path-grouping condition alongside spring-boot. spring-cloud was deliberately left out of this condition since useTags defaults to false there and changing it would alter existing default grouping behavior for that library.
• Add regression tests covering useTags=false/true grouping for both spring-http-interface and spring-declarative-http-interface.
• Update two pre-existing kotlin-spring tests whose expected output filenames assumed the old (buggy) tag-based grouping for spring-declarative-http-interface.

PR checklist

• Read the contribution guidelines.
• Run the following to build the project and update samples:

  ./mvnw clean package || exit
  ./bin/generate-samples.sh ./bin/configs/*.yaml || exit
  ./bin/utils/export_docs_generators.sh || exit
  

  (For Windows users, please run the script in WSL)
  Commit all changed files.
  This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master.
  These must match the expectations made by your contribution.
  You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*.
  IMPORTANT: Do NOT purge/delete any folders/files (e.g. tests) when regenerating the samples as manually written tests may be removed.
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.

---

Summary by cubic

Fixes useTags=false being ignored for Spring HTTP interface libraries so operations group by first path segment instead of tag.

• Adds spring-http-interface and spring-declarative-http-interface to the path-grouping condition.
• Sanitizes path segments into valid Java identifiers (e.g. another-fake becomes anotherFake), prefixes digit-leading segments with Class, and falls back to Path for segments that sanitize to empty (e.g. @ or !), with root paths grouped under default.
• Keeps groups unique when sanitization causes collisions by appending a numeric suffix (e.g. anotherFake2, Path2).
• Leaves spring-cloud unchanged because useTags defaults to false there, and altering it would change existing grouping behavior.
• Updates petstore samples and sample configs to set useTags: "true" for spring-http-interface, which also renames FakeClassnameTags123Api to FakeClassnameTestApi and moves a few operations between FakeApi and PetApi.

^{Written for commit 70948e0. Summary will update on new commits.}