ci: Remove volatile metadata from generated models header (#718)

## Summary

- Disable `timestamp` and `filename` in the `datamodel-codegen` header
via `pyproject.toml` config
- Prevents spurious PRs when the OpenAPI spec hasn't changed but
regeneration produces a diff due to volatile metadata (timestamp, source
URL).

## Test plan

- [x] Regenerated models locally — header is now a single static line `#
generated by datamodel-codegen`
- [x] CI passes

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: vdusek

pyproject.toml

@@ -226,6 +226,8 @@ extra_fields = "allow"
 allow_population_by_field_name = true
 aliases = "datamodel_codegen_aliases.json"
 formatters = ["ruff-check", "ruff-format"]
+custom_file_header = "# generated by datamodel-codegen"
+disable_timestamp = true
 
 [tool.uv]
 # Minimal defense against supply-chain atatcks.

src/apify_client/_models.py

@@ -1,6 +1,4 @@
-# generated by datamodel-codegen:
-#   filename:  openapi.json
-#   timestamp: 2026-04-13T07:02:33+00:00
+# generated by datamodel-codegen
 
 from __future__ import annotations