fix(llm): make ProviderClient.chat attempt count match MAX_RETRIES

[Arvuno]

fix(llm): make ProviderClient.chat attempt count match MAX_RETRIES

ProviderClient.chat used while retries <= MAX_RETRIES: with
retries = 0, so an operator setting MAX_RETRIES=3 actually triggered
four LLM calls. The final log line printed MAX_RETRIES rather than the
attempt count, hiding the off-by-one behind a misleading number.

Replace the while loop with for attempt in range(1, MAX_RETRIES + 1):.
The body no longer carries a manual retries counter and the
last_error variable is properly typed. The final error log now reports
the actual number of attempts ("after N attempt(s)") and the
ProviderError that gets raised still carries status_code 502.

Adds backend/tests/test_provider_client_retry.py with two cases:

• litellm.completion fails once then succeeds → exactly 2 calls.
• litellm.completion always fails → exactly MAX_RETRIES calls, the
  final error log contains "attempt" and no longer contains "retries".