fix(rates): repoint rate_finder to renamed hummingbot token normalizer

[fengtality]

Fixes #191

Problem

HAPI fails at startup with ModuleNotFoundError: No module named 'hummingbot.core.gateway.utils' when built against hummingbot development newer than June 28. Hummingbot commit a71a92a5 deleted hummingbot/core/gateway/utils.py and renamed the helper unwrap_token_symbol → normalize_token_symbol, relocating it to hummingbot.core.rate_oracle.utils. utils/rate_finder.py (introduced in f067c37) still imported from the deleted path, killing the whole app import chain (main.py → routers → deps → services → rate_finder).

Changes

1. Import fix: rate_finder.py now imports normalize_token_symbol from hummingbot.core.rate_oracle.utils (its new name and location).
2. Latent rate-resolution bug found during validation: the find_rate port never re-checked the direct pair after USD→USDT normalization, so e.g. HBOT-USD returned None even with HBOT-USDT present in the ticker pool. Added the normalized-pair re-check to match upstream hummingbot's find_rate.

Compatibility note for QA

This PR requires a hummingbot wheel built from development at/after a71a92a5 (the module rename). It will not import against older wheels (e.g. 20260515), which only have the old unwrap_token_symbol path — pair this with a current dev wheel when testing.

⚠️ Wheel-build gotcha that masked this bug: setuptools' incremental build/lib never propagates source-file deletions, so wheels rebuilt from an existing hummingbot checkout silently re-include the deleted core/gateway/utils.py (and three stale hummingbot/cli/ files). If a rebuilt wheel still "works" with the old import, check unzip -l dist/hummingbot-*.whl | grep gateway/utils and purge stale files from build/lib.* (or delete the build dir) before building.

Testing

• Reproduced the exact ModuleNotFoundError from the issue by importing rate_finder against post-a71a92a5 hummingbot source.
• After the fix, with a clean-built current dev wheel installed:
  • main.py (the uvicorn entrypoint) imports cleanly end-to-end.
  • 9-case find_rate matrix passes: direct, direct-after-normalization (HBOT-USD), reverse, reverse-after-normalization, both cross-rate bridging paths, identity after normalization (USD-USDT → 1), and no-path → None.
• Swept every from hummingbot... import across routers/services/utils/models against the new layout — hummingbot.core.gateway.utils was the only break.

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