fix(functions): add configurable timeout and normalize abort/timeout errors as FunctionsFetchError

[mandarini]

🔍 Description

This PR addresses issue #1399 by adding a configurable client-side timeout for Functions invocations and by normalizing abort/timeouts into a typed FunctionsFetchError. Callers can now increase the client timeout when calling supabaseClient.functions.invoke(...) and reliably detect timeout/abort failures across environments.

What changed?

• Added an optional timeout parameter to the Functions client invocation (invoke) so callers can control how long the client will wait before aborting the request.
  • Usage: supabaseClient.functions.invoke('fn-name', { /* ... */, timeout: <number in ms> })
• When a request is aborted or hits the configured timeout, the Functions client now throws a FunctionsFetchError with a standardized kind (e.g. "timeout" or "abort").
  • The original error is preserved as the cause where supported, and message/name/stack are retained.
• Existing non-abort/fetch errors are left unchanged.
• Unit tests added for:
  • invoking with a short timeout -> throws FunctionsFetchError of kind "timeout"
  • aborted request via AbortController -> throws FunctionsFetchError of kind "abort"
  • non-abort network errors remain unchanged

Why was this change needed?

Issue #1399 reported that long-running edge functions were being cut off by a hard 60s client timeout and users had no way to increase it. Adding a user-configurable timeout fixes that UX gap. At the same time, different environments surface abort/timeouts with different error shapes (DOMException, AbortError, etc.), which makes it hard to handle these failures consistently. Normalizing those into FunctionsFetchError with an explicit kind gives consumers a single, stable error to catch and inspect.

Closes #1399

📝 Additional notes

• Backwards-compatible: API surface is extended with an optional timeout option; no breaking changes to existing signatures.
• Recommend documenting the new timeout option in the Functions client docs and showing an example for both browser and React Native usage.
• Tests are included to validate behavior across abort and timeout scenarios.