fix(responses): recover from malformed function call arguments

[gruffwy]

Problem

A malformed historical Responses function_call.arguments value currently rejects the entire follow-up request. Malformed ordinary tool-call arguments can also reach generated non-streaming or streaming Responses output.

A real failure used an argument string similar to:

{"path":"x","error_retry:: 240}

Behavior

• Skip a malformed historical ordinary function call and its matching output so later conversation turns can continue.
• Drop malformed ordinary tool calls from non-streaming Responses output.
• Validate accumulated ordinary tool-call arguments before streaming terminal events are emitted.
• Preserve custom-tool behavior, whose input is not governed by ordinary function-call JSON rules.
• Normalize empty ordinary arguments to {}.

The implementation deliberately does not guess how to repair malformed JSON.

Tests

• go test ./internal/translate ./internal/gateway
• GOFLAGS=-buildvcs=false go test ./internal/app -run 'TestImportConstraints|TestDutyBoundaries' -count=1
• git diff --check

The architecture test needed -buildvcs=false in this Windows worktree because Go VCS stamping could not inspect the linked worktree metadata.