Fix linter error EM101 and remove suppression from pyproject.toml

[Copilot]

EM101 (exception must not use a string literal) was suppressed globally in pyproject.toml rather than fixed. This removes the suppression and resolves all 18 violations across the codebase.

Changes

• All source files: Assign exception message strings to a msg variable before raising, per EM101 convention:

  # Before
  raise ValueError("Reusable taskflows can only contain 1 task")
  
  # After
  msg = "Reusable taskflows can only contain 1 task"
  raise ValueError(msg)

  Affected: _stream.py, capi.py, mcp_servers/codeql/client.py, mcp_transport.py, models.py, runner.py, sdk/copilot_sdk/backend.py, tests/test_sdk_copilot_adapter.py

• pyproject.toml: Remove EM101 from the ignore list.

• client.py (bonus fix): "Unsupported output format {fmt}" was missing the f prefix — fmt was never interpolated. Changed to f"Unsupported output format {fmt}".