fix(llm): auto-shape multimodal mediaPath messages in chat template

[msluszniak]

Description

LLMController.generate() collected imagePaths from messages with a mediaPath set, but never transformed their content into the [{type:'image'}, {type:'text', text}] form that the chat template needs to emit the <image> placeholder. Calling generate() directly with a vision-capable model (e.g. LFM2-VL) thus threw "More images paths provided than '<image>' placeholders in prompt" from native, even though sendMessage() worked because it built its own historyForTemplate that did the transformation.

This PR moves the transformation into applyChatTemplate so both call sites (generate and sendMessage) get the correct behavior, and removes the now-redundant historyForTemplate block from sendMessage. The public Message.content type stays string — external callers always pass plain strings; the controller handles the structured array form internally.

The helper is idempotent: messages whose content is already an array (e.g. callers who pre-shaped it as a workaround) are passed through unchanged.

Introduces a breaking change?

• Yes
• No

Public types are unchanged. sendMessage produces an identical rendered chat-template string (the transformation just happens one step later in the pipeline; token count and rendered output are byte-identical). generate only changes behavior in cases that previously threw — pure bug fix.

Type of change

• Bug fix (change which fixes an issue)
• New feature (change which adds functionality)
• Documentation update (improves or adds clarity to existing documentation)
• Other (chores, tests, code style improvements etc.)

Tested on

• iOS
• Android

The original bug was reproduced on a vision-capable model (LFM2-VL-1.6B-quantized) on Android while building a downstream consumer app. Re-verification of the fix on a real device is recommended before merge — see Testing instructions below. I have not personally re-run the failing scenario after the fix.

Testing instructions

To reproduce the original bug (without this PR):

import { LLMModule, LFM2_VL_1_6B_QUANTIZED } from 'react-native-executorch';

const llm = await LLMModule.fromModelName(LFM2_VL_1_6B_QUANTIZED);
await llm.generate([
  { role: 'user', content: 'Describe this image.', mediaPath: 'file:///path/to/image.jpg' },
]);
// Throws: "More images paths provided than '<image>' placeholders in prompt"

With this PR applied, the same call should succeed and return the model's description.

Regression check: a vision-capable sendMessage(text, { imagePath }) flow should continue producing identical output.

Screenshots

N/A (controller change, no UI).

Related issues

Addresses items 1 and 2 of #1086. With item 1 fixed, item 2's Message.content type mismatch no longer surfaces in practice because external callers never need to construct the array form themselves (the as unknown as string workaround that motivated #2 becomes unnecessary).

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 messagesForChatTemplate helper lives at module scope rather than as a static class method because it doesn't depend on controller state. Internal any[] return is a deliberate concession to the dynamic shape the chat-template engine accepts; the public Message[] input/output contract stays well-typed.