fix: Allow specifying lang code in chat requests

[Ruari-Phipps]

Summary

Adds --lang, --input-lang, and --output-lang flags to poly chat, allowing users to specify language codes for ASR (input) and TTS (output) when starting or continuing a chat session.

Motivation

Users chatting against multilingual agents need a way to specify the expected input/output language without relying on the project default. This exposes the existing asr_lang_code / tts_lang_code API fields via the CLI.

Changes

• Added --lang, --input-lang, --output-lang arguments to the chat subcommand in cli.py
• --lang sets both input and output lang; --input-lang/--output-lang override individually
• Threaded input_lang / output_lang through AgentStudioProject.create_chat_session, send_message, AgentStudioInterface, and PlatformAPIHandler for both standard and draft/branch chat flows
• Maps input_lang → asr_lang_code and output_lang → tts_lang_code in API request payloads

Test strategy

• Added/updated unit tests
• Manual CLI testing (poly <command>)
• Tested against a live Agent Studio project
• N/A (docs, config, or trivial change)

Checklist

• ruff check . and ruff format --check . pass
• pytest passes
• No breaking changes to the poly CLI interface (or migration path documented)
• Commit messages follow conventional commits

Screenshots / Logs

[github-actions]

Coverage Report

Base (main)	PR	Change
71.6%	71.5%	-0.2% ⚠️

Changed file coverage

File	Coverage	Change
poly/handlers/platform_api.py	32.1%	-4.5% ⚠️
poly/cli.py	43.5%	+0.1% ✅