Enforce expected_runtime and confidence for executable model responses

[CoreyRDean]

Motivation

• The v1 spec requires executable proposals to include both an expected runtime and the model's confidence, and the codebase needed to consistently enforce that contract.
• Strengthen defense-in-depth by validating those fields both in the JSON schema sent to backends and at runtime so backends that bypass schema still fail closed.

Description

• Tighten the JSON schema in internal/model/schema.go to require expected_runtime and confidence for approach=command and approach=script branches.
• Add presence checks for ExpectedRuntime and Confidence in Response.Validate() in internal/model/types.go so runtime validation matches the schema.
• Update the mock backend in internal/model/mock/mock.go to populate ExpectedRuntime for the INTENT_MOCK_CMD override and sample responses so tests remain representative.
• Add unit tests in internal/model/types_test.go that assert missing expected_runtime and confidence are correctly rejected for both command and script approaches.

Testing

• Ran the focused packages tests with go test ./internal/model/... ./internal/cli/... which completed successfully.
• Ran the full unit suite with go test ./... which completed successfully (all tests passed or reported as expected/cached).
• The newly added internal/model/types_test.go passed as part of the above runs.

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Codex Task