fix: preserve temperature=0.0 and max_tokens=0 in subgrammar() factory

[alvinttang]

Summary

guidance._grammar.subgrammar() used if temperature: / if max_tokens: to decide whether to attach those kwargs to the built RuleNode. Because 0.0 and 0 are falsy in Python, callers passing temperature=0.0 (the canonical way to ask for deterministic / greedy sampling) or max_tokens=0 had their values silently dropped — the resulting grammar had no RuleNode.temperature or RuleNode.max_tokens set.

Every other factory in the codebase (guidance.json, guidance._grammar.gen) already uses is not None for the same kwargs, so subgrammar() was the only inconsistent path.

Root cause

# guidance/_grammar.py (before)
if max_tokens:          # wrong: 0 is valid
    body = token_limit(body, max_tokens=max_tokens)
if temperature:         # wrong: 0.0 is valid
    body = with_temperature(body, temperature=temperature)

Python treats 0 and 0.0 as falsy, so the guards silently drop the valid-but-falsy values before they reach the RuleNode.

Fix

Two-line change in guidance/_grammar.py: swap both guards for is not None, matching the rest of the codebase:

if max_tokens is not None:
    body = token_limit(body, max_tokens=max_tokens)
if temperature is not None:
    body = with_temperature(body, temperature=temperature)

Tests

tests/unit/library/test_subgrammar.py adds two parallel test classes:

• TestSubgrammarFactoryTemperature: verifies temperature=0.0 propagates (RED on main), temperature=0.5 still works, temperature=None sets nothing.
• TestSubgrammarFactoryMaxTokens: same three cases for max_tokens=0 / max_tokens=16 / max_tokens=None.

A helper walks the resulting grammar tree via GrammarNode.children() and collects every RuleNode.temperature (or max_tokens) that is not None.

Locally on darwin/arm64:

• pytest tests/unit/library/test_subgrammar.py → 14 passed.
• Full unit suite → 1582 passed, 1 skipped, 220 xfailed (all pre-existing; no new failures).
• ruff check clean.

Risk notes

• Behaviour change: callers who were passing temperature=0.0 or max_tokens=0 expecting them to be ignored will now see them forwarded to llguidance. llguidance accepts max_tokens=0 cleanly (verified locally via LLMatcher smoke test) — max_tokens=0 is also already the honoured value in gen() / json() / lark() per _ast.py serialization, so this just brings subgrammar() in line.
• None (the documented default) still means "unset" and sets no limit. That invariant is covered by the _none_sets_nothing tests.
• No API shape change.

Related to the long-standing observation in #600 that temperature can be silently dropped on certain call paths; this PR addresses the subgrammar() path specifically.

[riedgar-ms]

I'm OK with this. @hudson-ai @nking-1 ?