fix: discriminate tool search result content

[MukundaKatta]

Summary:

• mark ToolSearchToolResultBlock.content as a discriminated union by type
• apply the same fix to the beta variant
• add regression tests to ensure nested tool search result content is parsed into typed models instead of raw dicts

Testing:

• attempted pytest for the new test file, but this local environment is missing the inline_snapshot test dependency from the repo test setup

Closes #1394