feat(solidjs): add experimental attachments support in useChat hook

[Jaikant]

Added experimental attachments support to SolidJS useChat hook by:

Mirroring React implementation's by:
Implementing prepareAttachmentsForRequest in:

• append() method
• handleSubmit() handler

Adding experimental_attachments to:

• Message object construction

Ensures feature parity with React implementation while maintaining SolidJS reactivity patterns.

[Jaikant]

Fixes #4565

[lgrammel]

Can you run prettier, add examples, add tests (see examples & tests in the React version for reference)?

[Jaikant]

Can you run prettier, add examples, add tests (see examples & tests in the React version for reference)?
@lgrammel Can you please check if it is okay now.

[lgrammel]

@Jaikant did you run and test out the example? the backend part seems missing

[Jaikant]

@lgrammel yes it works. The same api works for the attachment as well, since it is added as part of the messages array. No other changes needed in the backend.

[lgrammel]

Great. Please run prettier; update the docs tables (see #4567 for example) and add a changeset (path for ui/solid).

[lgrammel]

pnpm prettier-fix

[Jaikant]

Weird because I had run that, maybe I missed something. Let me check my workspace again.

[Jaikant]

@lgrammel Done. Please check.