fix(llm): normalize multimodal image paths to file:// URIs

[msluszniak]

Description

LLMController.forward passed imagePaths straight through to nativeModule.generateMultimodal with no normalization. The native side requires the file:// prefix; without it, native throws "Read image error: invalid argument" with no further context. Callers can plausibly arrive with either form:

• ResourceFetcher.fetch returns raw paths without file:// (per its own docstring on the fetch method).
• Platform image-picker APIs (e.g. expo-image-picker) typically return file:///... URIs.
• The same path string passed to a vision module's forward(...) works either way; the asymmetry between vision modules and multimodal LLM is undocumented.

This PR normalizes each image path inside LLMController.forward so both forms work, and updates the JSDoc on Message.mediaPath and LLMModule.forward.imagePaths to document the new contract.

Introduces a breaking change?

• Yes
• No

Strictly additive: previously-working calls (paths with file://) keep working unchanged. Previously-failing calls (paths without file://) now succeed.

Type of change

• Bug fix (change which fixes an issue)
• New feature
• Documentation update
• Other

Tested on

• iOS
• Android

The bare-path failure was reproduced on Android (Samsung Galaxy S24 Ultra) with LFM2-VL-1.6B while building a downstream consumer; both forms tested manually post-fix on the same device. Re-verification of both forms on iOS is recommended.

Testing instructions

import { LLMModule, LFM2_VL_1_6B_QUANTIZED } from 'react-native-executorch';
const llm = await LLMModule.fromModelName(LFM2_VL_1_6B_QUANTIZED);

// Both should now work; previously only the first did.
await llm.generate([
  { role: 'user', content: 'Describe.', mediaPath: 'file:///absolute/path/to/img.jpg' },
]);
await llm.generate([
  { role: 'user', content: 'Describe.', mediaPath: '/absolute/path/to/img.jpg' },
]);

Related issues

Addresses item 3 of #1086.

Checklist

• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have updated the documentation accordingly
• My changes generate no new warnings

Additional notes

The normalizer is module-scope (matching messagesForChatTemplate from #1089) rather than a class method because it doesn't depend on controller state.