feat: Add apify.errors domain-level error taxonomy

[vdusek]

Introduces a machine-readable, domain-level error taxonomy in a new apify.errors module. Each error carries a stable code and a retryable flag so automated (agent) callers can branch on a failure without parsing log messages.

The module adds ApifyError(code, retryable) and the typed subclasses ActorRunError, ActorTimeoutError, InputValidationError, ChargeLimitExceededError, AuthenticationError, and RateLimitError. ApifyError.from_client_error() maps apify_client's HTTP-status-based exceptions to these at the SDK boundary, and ActorRunError.from_run() derives the right run error from a terminal run status. The names are exported additively from apify/__init__.py.

The change is fully additive: no existing method changes what it raises, and InputValidationError subclasses ValueError so except ValueError still catches it. The mapping helpers are available for new code paths (e.g. opt-in raise_on_failure) but are not wired into any existing method.

Note: ActorTimeoutError subclasses ActorRunError (rather than ApifyError directly as sketched in the issue) so it reuses the run-based constructor and carries .run_id/.status/.exit_code/.status_message, while still being an ApifyError.

Closes #988

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.51%. Comparing base (e8cda0a) to head (082f090).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #990      +/-   ##
==========================================
+ Coverage   90.35%   90.51%   +0.16%     
==========================================
  Files          49       50       +1     
  Lines        3089     3153      +64     
==========================================
+ Hits         2791     2854      +63     
- Misses        298      299       +1     

Flag	Coverage Δ
e2e	36.72% <60.93%> (+0.50%)	⬆️
integration	57.50% <60.93%> (+0.03%)	⬆️
unit	79.35% <100.00%> (+0.42%)	⬆️

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