PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• 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.
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Summary by cubic

Fixes Kotlin string escaping across generators and hardens Markdown rendering to prevent broken docs. Prevents interpolation and compile errors for $, backslashes, quotes, newlines, triple‑quoted strings, and comment‑close sequences in paths, annotations, docs, and defaults.

• Bug Fixes
  • Expose raw fields for templates: unescapedSummary, unescapedNotes, unescapedMessage, unescapedDescription, unescapedDefaultValue (ops, params, properties, responses).
  • Add Mustache lambdas under lambda.*: escapeInNormalString, escapeDollarInMultiline, escapeDollar, removeLineBreak; add lambda.escapeMarkdown in DefaultCodegen; remove old per‑generator lambdas in KotlinSpringServerCodegen.
  • Apply escaping to annotation strings and paths across kotlin-spring, kotlin-client, kotlin-server, kotlin-misk, kotlin-wiremock (Retrofit2/@get and @Query/@Path/@Header, OkHttp, Volley, Ktor2, Vertx, JAX-RS, Javalin 5/6, Multiplatform, Spring RestClient/WebClient). @JsonProperty names and PATH const vals now escape $.
  • Render string defaults from unescapedDefaultValue with escapeInNormalString in params and model properties; keep enums and numbers unchanged.
  • Use unescapedSummary/unescapedNotes and unescapedDescription in docs and OpenAPI annotations; escape multiline descriptions with escapeDollarInMultiline; escape Markdown in API/class docs with lambda.escapeMarkdown to avoid premature code block termination.
  • Add worst‑case spec and tests (issue 20502), new Echo API samples covering $ in path/query and defaults; tests assert @JsonProperty and PATH const string escaping; update kotlin-misk configs to use petstore-kotlin-misk.yaml; regenerate samples and docs.

^{Written for commit 779bcab. Summary will update on new commits. Review in cubic}