fix(parsing): vLLM-parity type ladder for XML tool-call arg coercion

[hallerite]

Summary

Closes #47. Renderers' XML-style parsers (Qwen3.5, GLM, MiniMax, Laguna) flagged <parameter=x>True</parameter> as INVALID_JSON for boolean params because json.loads("True") fails — even though both SGLang's Qwen3CoderDetector and vLLM's Qwen3CoderToolParser accept Pythonic literals via case-folded comparison. Models freely emit True/False at inference because the reference parsers normalize them, but the same outputs came back malformed through renderers.

This swaps _coerce_arg_value from a string-or-json.loads dispatch to the full _convert_param_value ladder both reference parsers share:

Declared type	Coercion
(none) — no schema passed	json.loads, fallback to raw text + INVALID_JSON (historical behavior preserved)
string / str / text / varchar / char / enum	return verbatim
int / uint / long / short / unsigned	int(text), fall back to string + INVALID_JSON
num / float	float(text) with SGLang's source-string int-demotion heuristic, fall back to string + INVALID_JSON
boolean / bool / binary	text.lower() == "true"; non-true/false → False + INVALID_JSON
object / array / dict* / list* / anyOf	json.loads, then ast.literal_eval fallback (Python literals like {'k': 1}), then string + INVALID_JSON
Unknown	ast.literal_eval, fall back to string + INVALID_JSON
null literal (any case)	None — except for declared string-family types (see deviation below)

INVALID_JSON is preserved as the verifier / RL-loss signal whenever the value had to degenerate (e.g. yes for a bool, abc for an int).

Deliberate deviation from vLLM/SGLang

Both reference parsers null-coerce "null" before checking type, so a string-typed arg of "null" returns Python None. Renderers keeps "null" verbatim for type: "string" so the existing tests/test_tool_arg_type_preservation.py::test_string_arg_preserves_type[*-string-null] contract holds across all five XML renderers. The XML wire format already can't distinguish the string "null" from JSON null, but when the schema explicitly says type: "string" we honour it.

Reference parser sources

• SGLang: python/sglang/srt/function_call/qwen3_coder_detector.py
• vLLM: vllm/tool_parsers/qwen3coder_tool_parser.py::_convert_param_value

The two are functionally near-identical for this code path; vLLM is slightly more permissive (extra anyOf → object branch). Renderers now matches vLLM's superset, with the one string-null deviation noted above.

Test plan

• pytest tests/test_arg_coercion.py — 43 new unit tests cover every branch of the ladder, including the issue's True/False regression through full parse_qwen35 round-trip.
• pytest tests/test_tool_arg_type_preservation.py — 30/30 string-preservation cases pass across all five XML renderers (the string-null case still round-trips verbatim).
• Full suite: 1371 passed, 53 skipped, 1 xfailed.

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