fix(stables): retry DeFiLlama price requests

[spalen0]

Summary

• route DeFiLlama current-price requests through the shared retrying HTTP client
• retry transient TLS/connection errors, timeouts, HTTP 429s, and 5xx responses
• remove the now-unused defillama-sdk dependency
• add regression coverage for recovery from an SSLError and price response parsing

Persistent failures still propagate after the retry budget is exhausted, preserving the existing crash alert.

Validation

• uv run --extra dev pytest tests/ -q — 986 passed, 6 skipped, 5 subtests passed
• uv run --extra dev ruff check utils/defillama.py tests/test_http_client.py tests/test_utils.py
• uv lock --check
• git diff --check

Fixes #371