fix(agent): recover concatenated tool arguments

[wolfkill]

Summary

• Fixes [Bug]: Failed to parse tool arguments: invalid character '{' after top-level value #1455.
• Recovers tool calls where the model/provider concatenates two JSON argument objects into one Function.Arguments string, e.g. {...}{...}.
• Uses only the first complete top-level JSON object, then keeps the existing RepairJSON fallback unchanged for other malformed JSON cases.
• Adds regression coverage for the Agent tool-call path and JSON object extraction edge cases.

Why this is safe

• Valid JSON arguments are not changed because this recovery only runs after normal json.Unmarshal fails.
• Existing JSON repair behavior is preserved when no complete first JSON object can be recovered.
• The fix does not merge multiple objects, which would be ambiguous; it executes the first valid argument payload and logs the recovery.

Tests

• CGO_LDFLAGS="-lc++" go test ./internal/agent -run TestRunToolCallRecoversConcatenatedJSONArguments -count=1
• CGO_LDFLAGS="-lc++" go test ./internal/agent/tools -run 'Test(ExtractFirstJSONObject|RepairJSON)' -count=1
• CGO_LDFLAGS="-lc++" go test ./internal/agent ./internal/agent/tools -count=1
• CGO_LDFLAGS="-lc++" go test ./internal/agent/... -count=1
• git diff --check

Also ran CGO_LDFLAGS="-lc++" go test ./internal/... -count=1; Agent-related packages passed, while unrelated existing tests failed in internal/application/service, internal/application/service/file, internal/datasource/connector/yuque, and internal/infrastructure/web_search.