Add information about .pyi to copilot-instructions.md (openvinotoolkit#34518)

### Details:
- Python stub files (`.pyi`) under `src/bindings/python/` are
auto-generated by a scheduled GitHub Actions workflow
(`update_pyapi_stubs.yml`) using `pybind11-stubgen`.
Comments requesting changes to it
[(example)](openvinotoolkit#34511 (comment))
are not necessary.


### AI Assistance:
 - *AI assistance used: yes*

Author: almilosz

.github/copilot-instructions.md

@@ -99,6 +99,7 @@ Before posting any comment, apply this gate:
 - Do not review vendored/third-party sources under `thirdparty/` unless the PR explicitly modifies integration or patch logic.
 - Do not enforce component-specific runtime behavior rules on docs-only PRs.
 - For generated or auto-updated files (for example stubs/version bumps), comment only if there is clear breakage risk.
+- Python stub files (`.pyi`) under `src/bindings/python/` are auto-generated by a scheduled GitHub Actions workflow (`update_pyapi_stubs.yml`) using `pybind11-stubgen`. Do not request manual fixes to stub files.
 
 ## Component-Specific Expectations