chore(ci): add Python 3.14 support

[barredterra]

Description

Add official Python 3.14 support by raising the requires-python upper bound and testing against 3.14 in the unit test CI matrix.

Changes:

• pyproject.toml: requires-python from >=3.11,<3.14 to >=3.11,<3.15
• .github/workflows/tests-unit.yaml: add 3.14 to the python-version matrix

No source code changes were required; existing tests pass on Python 3.14.

PR Type

• Infrastructure

Relevant issues

N/A

Checklist

• I understand the code I am submitting.
• I have added unit tests that prove my fix/feature works.
• I have run this code locally and verified it fixes the issue.
• New and existing tests pass locally.
• Documentation was updated where necessary.
• I have read and followed the contribution guidelines.
• AI Usage:
  • No AI was used.
  • AI was used for drafting/refactoring.
  • This is fully AI-generated.

AI Usage Information

• AI Model used: Composer 2.5 (Cursor)
• AI Developer Tool used: Cursor
• Any other info you'd like to share: Used to identify required changes for Python 3.14 compatibility and draft the PR description.

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Resolves #16