feat: add timeout support to RequestContext and isomorphic-fetch

[dahaque]

This PR adds first-class timeout support to the typescript-fetch generator so users can prevent requests from hanging indefinitely.

Why?

The current typescript-fetch client doesn’t offer any way to cancel or fail a request after a specified time.
When Kubernetes or any API becomes slow, consumers have no built-in way to enforce a timeout leading to stalled network calls.

This PR introduces a simple, lightweight solution.

What this PR does

1. Adds a withTimeout() wrapper
   -Wraps fetch with an AbortController
   -Automatically aborts the request when timeout limit is reached
   -Forwards user-provided abort signals correctly

2. Extends RequestContext
   -Adds a timeout?: number property
   -Ensures timeout travels through the request flow cleanly

3. Integrates timeout into the generated IsomorphicFetchHttpLibrary
   -Replaces direct use of fetch
   -Uses withTimeout(fetch, request.getTimeout())
   so all requests automatically inherit the timeout setting

[wing328]

thanks for the PR

what about adding a docstring explaining what this function does?

please also follow step 3 in the PR checklist to update the samples so that CI can verify the change

[macjohnny]

please specify what unit the number is, also in the modules/openapi-generator/src/main/resources/typescript/http/http.mustache

[wing328]

cc @TiFu (2017/07) @taxpon (2017/07) @sebastianhaas (2017/07) @kenisteward (2017/07) @Vrolijkx (2017/09) @macjohnny (2018/01) @topce (2018/10) @akehir (2019/07) @petejohansonxo (2019/11) @amakhrov (2020/02) @davidgamero (2022/03) @mkusaka (2022/04) @joscha (2024/10)