[typescript-fetch] centralise date handling, add dateLibrary

[wing328]

based on #24637 with updated samples.

all TS tests passed.

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
  

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  Commit all changed files.
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  These must match the expectations made by your contribution.
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  IMPORTANT: Do NOT purge/delete any folders/files (e.g. tests) when regenerating the samples as manually written tests may be removed.
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Summary by cubic

Centralizes date handling for typescript-fetch and adds a dateLibrary option to choose Date (default) or string. Fixes format: date bugs (incorrect form param serialization and UTC day shifts) by using the local calendar for date and UTC for date-time.

• Runtime: all date parsing/serialization now uses parseDate, parseDateTime, serializeDate, and serializeDateTime in runtime.ts; querystring and path/query/form params route through these.
• Generator: new additionalProperties.dateLibrary option (date | string); withoutRuntimeChecks implies string. Docs updated.
• Templates: models and oneOf guards align with helpers; date imports only when needed via x-hasDateVars. ES6-safe code; preserves years 0000–0099 and rejects invalid dates.
• Tests/samples: added a fixture covering all date locations and two sample builds (date and string).

Migration

• If your client expects string dates, set -p dateLibrary=string (or dateLibrary: string in config). Regenerate and update types accordingly.
• If you relied on withoutRuntimeChecks for string dates, no change; to get Date, disable that flag and/or set -p dateLibrary=date.

^{Written for commit b71f03e. Summary will update on new commits.}