feat: improve token validation with exponential backoff and add unit tests

[travellingsoldier85]

…tests

• Refactor token_mgmt.py with exponential backoff retry logic
• Add validate_token() returning diagnostic info (username, rate limits)
• Handle specific exceptions (Timeout, ConnectionError) instead of bare Exception
• Add rate limit monitoring with low-remaining warnings
• Add 12 unit tests for token_mgmt module
• Add 13 unit tests for utils module (mask_secret, parse_repo_name)
• All existing tests continue to pass

Summary

Improves miner token management reliability and adds comprehensive unit test coverage for two previously untested modules.

Changes

gittensor/miner/token_mgmt.py — Improved token validation:

• Added validate_token() function that returns diagnostic info (authenticated username, rate limit status)
• Replaced fixed 3s retry sleep with exponential backoff (2s → 4s → 8s)
• Handle specific exceptions (Timeout, ConnectionError, RequestException) instead of bare Exception
• Immediate failure on HTTP 401 (no wasteful retries for revoked tokens)
• Rate limit detection: warns when API calls are running low, reports reset time on 403
• is_token_valid() preserved as backward-compatible convenience wrapper

tests/miner/test_token_mgmt.py — 12 new tests:

• Valid/invalid/rate-limited token scenarios
• Retry behavior on timeouts and connection errors
• Exponential backoff timing verification
• No-retry on definitive 401 failures

tests/utils/test_utils.py — 13 new tests:

• mask_secret(): format, length, determinism, edge cases
• parse_repo_name(): parsing, case normalization, error handling

Testing

All 356 tests pass (331 existing + 25 new).

Related Issues

Type of Change

• Bug fix
• New feature
• Refactor
• Documentation
• Other (describe below)

Testing

• Tests added/updated
• Manually tested

Checklist

• Code follows project style guidelines
• Self-review completed
• Changes are documented (if applicable)