fix: handle Cursor Auto (default) and cursor-grok reasoning models

[cosmyo]

Problem

Two related bugs break Cursor models when Pi has a non-off default thinking level (e.g. defaultThinkingLevel: "medium" in settings):

1. Cursor Auto (default)

supportsReasoningModelId("default") returned true, so with defaultThinkingLevel: "medium" the proxy rewrote default → default-medium. Cursor rejects that model id with Provider finish_reason: error.

Repro

1. Set defaultProvider: "cursor", defaultModel: "default", defaultThinkingLevel: "medium"
2. Start Pi and send any prompt
3. First request fails with responseModel: "default-medium"

2. cursor-grok-4.5 (reasoning / effort-only model)

Cursor exposes only effort-suffixed variants (cursor-grok-4.5-low, -medium, -high). After dedup they collapse to cursor-grok-4.5 with an effort map, but:

• supportsReasoningModelId() did not recognize the cursor-grok-* prefix (regex expects bare grok-)
• effort mapping lived in compat.reasoningEffortMap, but Pi reads model.thinkingLevelMap
• without effort suffix, requests sent bare cursor-grok-4.5, which is invalid

Fix

• supportsReasoningModelId: return false for default; recognize cursor-grok-*
• modelConfig: set thinkingLevelMap from deduped effort map; mark effort-deduped models as reasoning: true
• resolveModelId: skip effort insertion for default; default bare cursor-grok-* to medium when no effort is provided

Tests

• Updated default reasoning expectation
• Added buildThinkingLevelMap coverage
• Added cursor-grok-4.5 dedup + thinking map tests
• Added resolveModelId tests for default and cursor-grok-*

All 93 tests pass locally.

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Note: upstream issues appear to be disabled, so this PR doubles as the bug report. Happy to split or adjust if there's a preferred reporting channel.