Harden assistant flow and OCR input handling

[tdtgit]

Motivation

• Prevent runtime failures from out-of-bounds Telegram photo access and from non-function or malformed model outputs.
• Make the assistant flow more resilient so tool dispatch and transaction parsing don't crash the service.

Description

• Add getLatestPhotoFileId(message) and use it in imageOcr to avoid fixed-index access like message.photo[3] and fail fast when no photo is present.
• Replace brittle access of response.output[0] with a search for a function_call item and handle the case where the model returns no function call by returning Request completed early.
• Use functionCall.arguments when dispatching tool handlers instead of assuming response.output[0].arguments.
• Add defensive JSON parsing in processTransaction using try/catch around JSON.parse to log and return early on malformed model output.

Testing

• Ran the test suite with bun test, all tests passed (3 pass, 0 fail).

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Codex Task