feat(ai): add UnsupportedOperation error for route capability mismatches

[rekram1-node]

Calling an operation the selected route does not implement (e.g. LLMClient.compact on Anthropic) failed with a generic InvalidRequest, forcing consumers to match on message text. This adds a typed distinction.

Name options considered

• UnsupportedOperation (chosen): matches the existing noun-phrase style (InvalidRequest, NoRoute, UnknownProvider) and reads naturally next to operation: "compact".
• UnsupportedCapability: arguably more precise (the failure is a missing route capability, detected before any attempt), but vaguer at the call site.
• OperationNotSupported / NotSupported: same meaning, less consistent with the current taxonomy.

Open to renaming before merge; the shape below stays the same either way.

Shape

AI.Error.UnsupportedOperation (packages/ai/src/schema/errors.ts) with operation: string plus optional provider/route. operation is an open string so future ops (video generation, etc.) reuse it without another union member. ProviderShared.unsupportedOperation({ operation, message, provider?, route? }) is the shared constructor.

Migrated (missing capability, never touches the network)

• route/client.ts: explicit compact on a route without a compact endpoint → operation: "compact"
• protocols/xai-responses.ts: automatic contextManagement on xAI (explicit-only route) → operation: "in-band-compaction"
• core/src/aisdk.ts: AI SDK bridge hitting native compaction replay state → operation: "compaction-replay"

Required follow-through: to-session-error.ts maps it to provider.unsupported-operation (open wire string, no schema/regen needed), retry.ts treats it as non-retryable like InvalidRequest/NoRoute.

Deliberately left as InvalidRequest

The other ~20 invalidRequest sites are malformed caller input, not missing capabilities (bad media/base64, missing filename, conflicting options, missing credentials, cross-provider compaction replay, non-object overlays). Those stay; the rule is: capability missing → UnsupportedOperation, input invalid → InvalidRequest.

Consumer guidance

Prefer the capability guard over try/catch: LLMClient.canCompact(request) narrows to CompactionRequest at both type and runtime level. Catch-and-fallback now has a real branch: reason._tag === "UnsupportedOperation" && reason.operation === "compact".

Verification

• bun typecheck clean in packages/ai (incl. tsconfig.types.json capability assertions) and packages/core
• packages/ai: full suite 980 pass / 0 fail; updated explicit-compaction, schema reason-enumeration, and added an Anthropic compact rejection test
• packages/core: session-error (mapping + non-retryable) and aisdk (replay rejection) pass