Add pytest coverage for core API

[Ludy87]

Motivation

• Provide a comprehensive pytest suite to validate core library behavior and enable reliable CI checks.
• Make tests runnable against the repository source layout without installing the package.

Description

• Add tests/conftest.py to prepend src to sys.path so tests can import the package layout directly.
• Add unit tests covering exceptions, models/serialization, enums/status values, GraphQL client (sync/async), HandlerGQL header/login/query wrappers, GQLHandler query/mutation wrappers, core PyXplora utilities, and sync PyXploraApi flows (fake GQL handler).
• Update .gitignore to allow committing tests/ while continuing to ignore test bytecode and caches.

Testing

• Ran the full test suite with pytest -q and all tests passed: 33 passed.
• Tests exercise both synchronous and asynchronous client helpers as well as serialization and header-building logic using lightweight fakes and monkeypatching.

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Codex Task

[sonarqubecloud]

Quality Gate failed

Failed conditions
1 Security Hotspot

See analysis details on SonarQube Cloud