This PR is **part 1 of 3** splitting
https://github.com/ComposioHQ/composio/pull/3505 to make the removal of
the old 2025 custom tools easier to review. It carries the **Python**
slice.
- removes the legacy `composio.tools.custom_tool` registry path:
`core.models.custom_tools`, `ExecuteRequestFn`, the old fallback
execution wiring, the security tests, and the example
- preserves the 2026 tool-router APIs: `composio.experimental.tool()`,
`composio.experimental.Toolkit`, inline custom-tool execution,
preload/attach/use flows, and `session.custom_tools()`
- bumps the Python workspace and all provider packages `0.13.1` →
`0.14.0`
## Notes
- This slice is a byte-identical subset of #3505 — the three split
branches recombine to that PR's exact tree. See #3505 for the original
local verification logs (`uv run --frozen nox -s chk`, targeted pytest);
CI re-runs per PR.
Alberto Schiabel