fix(variants): drop redundant thinking variant when effort enum present

[justin-carper]

Problem

For Anthropic/Claude models accessed via the Cursor provider, the Cursor SDK catalog exposes two reasoning params on the same model:

• a boolean thinking = ["false","true"]
• an effort enum, e.g. ["low","medium","high","xhigh","max"]

buildModelVariants emitted a variant for each, so opencode's variant cycler showed:

thinking + low / medium / high / xhigh / max

Standard opencode providers (driven by models.dev reasoning_options of type effort) show only the five effort levels — no thinking entry. The extra thinking variant was inconsistent and redundant (selecting any effort level already enables reasoning).

Fix

buildModelVariants (src/model-variants.ts):

• Added a hasEffortEnum pre-pass — scans params for any non-boolean reasoning enum, order-independent.
• Guarded the boolean reasoning branch: when hasEffortEnum is true, skip emitting the param-named boolean variant.
• Effort variants are unchanged — no thinking:"true" baked in (effort alone enables reasoning, per design decision).

Behavior matrix

Model param shape	Before	After
boolean thinking + effort enum (Claude via Cursor)	thinking + low/…/max	low/…/max ✅ parity
boolean thinking only (no enum)	thinking	thinking (unchanged)
effort enum only	effort levels	effort levels (unchanged)
fast toggle	fast opt-in + baked-off defaults	unchanged

Out of scope: composer-2.5 thinking=["off","on"] (treated as enum), fast, defaultModelParams.

Tests

Updated the former dual-param test and added 4 new cases in test/model-variants.test.ts:

1. Production claude-via-Cursor shape — thinking + effort + fast: asserts thinking suppressed, fast:"false" baked into effort variants, fast opt-in present.
2. Order-independence — effort enum declared after boolean still suppresses it.
3. Zero-value effort enum guard — empty enum does not count; boolean thinking survives.
4. Boolean without true — thinking=["false"] + effort → only effort variants.
5. Mixed boolean+enum param pinned — ["false","true","high"] current behavior pinned so a future catalog change is caught.

Verification

• npx vitest run → 190 passed (18 files)
• npx tsc --noEmit → exit 0

Review

Reviewed by code-reviewer agent: no critical issues, fix correct for all real catalog shapes. Tests added in response to its findings (production-shape+fast composition, zero-value-enum guard).