fix: clearer error message when remote functions are not enabled

[FrankFMY]

Summary

When submitting to a form remote function without having remote functions enabled in svelte.config.js, the error message was confusing:

POST method not allowed. No form actions exist for the page at /

This gives no indication that the user needs to enable remote functions. Now in dev mode, the error messages suggest enabling config.kit.experimental.remoteFunctions.

Before

[405] POST /
Error: POST method not allowed. No form actions exist for the page at /

After

Depending on the code path:

POST method not allowed. No form actions exist for the page at /. 
If you're trying to use remote functions, make sure they are enabled 
via `config.kit.experimental.remoteFunctions` in svelte.config.js

or for requests that reach the remote handler:

No remote functions were found. Make sure remote functions are enabled 
via `config.kit.experimental.remoteFunctions` in svelte.config.js

Changes

Improved error messages in four locations:

• handle_action_json_request in actions.js — JSON form submission without actions
• handle_action_request in actions.js — regular form submission without actions
• handle_remote_form_post_internal in remote.js — remote form submission with empty remotes
• handle_remote_call_internal in remote.js — remote function call with empty remotes

All hints are dev-mode only and don't affect production error messages.

Closes #14465

Test plan

• All 417 unit tests pass
• All 312 form/action integration tests pass (2 unrelated symlink failures)
• Error messages only appear in DEV mode
• Production error messages remain unchanged

[changeset-bot]

🦋 Changeset detected

Latest commit: 88aeec1

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@sveltejs/kit	Patch

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-kit-15268-svelte.vercel.app/

[dummdidumm]

Closing since the related issue was closed / deemed obsolete