feat(session): honor request chat_template_kwargs in local render and outbound

[guapisolo]

Depends on: #1819
Stacked on the parent PR's branch. Review / merge the parent first.

Summary

One merged chat_template_kwargs set drives both local input_ids render and the SGLang request.

Motivation

The session server forwarded request chat_template_kwargs to SGLang but rendered input_ids with the startup tokenizer's kwargs, so a client setting its response mode split the local token stream from SGLang's parser view. Both sides must resolve the response mode from the same kwargs.

Usage

POST /sessions/{session_id}/v1/chat/completions
{"messages": [...], "chat_template_kwargs": {"enable_thinking": false}}

The request value is merged over the launch-time kwargs; the merged set renders the local input_ids, the same set replaces the outbound chat_template_kwargs, a value conflicting with the family's fixed kwargs returns 400.

Design Notes

• Key choices: chat_completions builds a request-scoped TITO tokenizer via clone_with_chat_template_kwargs, which re-runs the family constructor over the same HF tokenizer with the merged kwargs.
• Re-running the constructor gives conflict validation for free; it also re-derives dependent kwargs such as DeepSeek's effective thinking.
• The startup singleton is never mutated; the clone is request-scoped.
• Accepted gaps are marked FIXME in core.py: top-level reasoning/reasoning_effort spellings are not mirrored into the kwargs, mid-session mode-change safety is a per-family property with no guard yet, compute_session_mismatch still renders with the startup tokenizer.

Verification

• Tests added: test_chat_template_kwargs_override_reaches_render_and_backend, test_chat_rejects_non_object_template_kwargs, test_chat_unknown_template_kwarg_passes_through in test_sessions.py; TestCloneWithChatTemplateKwargs in test_tito_tokenizer.py (113 passed at this commit).

Review Focus

• Scrutinize the kwargs merge block in core.py chat_completions for request spellings that bypass the nested object.
• Scrutinize clone_with_chat_template_kwargs for family state the constructor does not re-pin.

[gemini-code-assist]

Caution

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