refactor(typescript-fetch): centralize date type handling in TypeScriptFetchClientCodegen

[Mattias-Sehlstedt]

There are several incentives to support better and consistent date handling for typescript-fetch #24714, #24637. I saw that they had to define the same thing over and over again when they were technically referring to the exact same thing.

I have gathered the definition in a single location so that any further change will be easier to follow and understand.

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.

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Summary by cubic

Centralizes date type handling in TypeScriptFetchClientCodegen to unify date/DateTime mapping and type checks. Behavior unchanged: with dateLibrary=date, both map to TypeScript Date; otherwise both map to string.

• Introduces constants DATE_TYPE, DATE_TIME_TYPE, and TS_DATE_TYPE; updates mappings to use them.
• Adds shared helpers isDateType and isDateTimeType; parameter/property/model checks now delegate to them.
• Minor cleanup: use isEmpty() for cm.imports in parseImports.
• Verify typescript-fetch samples produce no generated diffs.

^{Written for commit 18b51b2. Summary will update on new commits.}

[cubic-dev-ai]

All reported issues were addressed across 1 file

<sub>Reply with feedback, questions, or to request a fix.

Re-trigger cubic</sub>