fix(ai): mark inception/mercury-2 thinkingLevelMap.off as null

Mercury 2 in instant mode (reasoning_effort: "none") disables tool calling.
The openai-completions provider hardcodes {reasoning:{effort:"none"}} when no
explicit reasoning level is passed and thinkingLevelMap.off isn't null
(openai-completions.ts:575), so every caller that doesn't opt in to a level
silently breaks Mercury 2's agentic use cases.

Setting thinkingLevelMap.off = null on the Mercury 2 catalog entry causes the
provider to omit the reasoning param entirely, letting Mercury 2's own default
take over. Low/medium/high pass through verbatim; OpenRouter normalizes them
to Mercury's vocabulary.

Prefix-matched on "inception/mercury-2" so future Mercury 2 variants on
OpenRouter inherit the fix.

packages/ai/src/models.generated.ts

@@ -9731,6 +9731,7 @@ export const MODELS = {
 			provider: "openrouter",
 			baseUrl: "https://openrouter.ai/api/v1",
 			reasoning: true,
+			thinkingLevelMap: {"off":null},
 			input: ["text"],
 			cost: {
 				input: 0.25,