test(01-02): add comprehensive unit tests for Tradier data layer
- 25 tests across 10 test classes covering all Phase 1 DATA requirements
- TestGetExpirations (DATA-02): DTE filtering, single-item normalization (Pitfall 5)
- TestGetOptionsChain (DATA-01): structure, values, single-contract normalization (Pitfall 2)
- TestGreeksPresent (DATA-03): all 1st-order Greeks populated
- TestGreeksAbsent (Pitfall 1): null Greeks handled gracefully
- TestIVPresent (DATA-04): bid_iv, mid_iv, ask_iv, smv_vol values
- TestDTEFilter (DATA-05): range filtering with expiration list update
- TestVendorRegistration (DATA-08): VENDOR_LIST, TOOLS_CATEGORIES, VENDOR_METHODS
- TestRateLimitDetection: HTTP 429 and X-Ratelimit-Available=0
- TestSessionCache: cache hit avoids redundant calls, clear forces re-fetch
- TestSandboxURL: env var toggles production vs sandbox URL
- All mocked, no real API calls; pytest added as dev dependency
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