Implement start_date in databento.fetch_daily_ohlc (was silently dead)

[mspinola]

Summary

• fetch_daily_ohlc's start_date parameter was never used on any code path: a cold cache always backfilled from 2000-01-01 (clamped to the GLBX.MDP3 floor 2010-06-06), and a warm cache always resumed from last_date + 1 day, regardless of what was passed. A caller trying to bound a fetch got a silent full-history pull at full Databento API cost instead.
• Implemented properly (revised after review -- the first version of this PR removed the parameter; that just relocates the problem to the next caller who wants a bounded fetch, and the mock-client test infrastructure to verify it correctly already exists in this file):
  • Cold cache (symbol's first-ever fetch): start_date now narrows the fetch floor (still clamped to the GLBX.MDP3 floor), so a narrow first-time query is actually cheap -- this is the real fix for the cost bug.
  • Warm cache (symbol already has history): start_date does not narrow the incremental fetch, which always resumes from the cache's own last_date + 1. The cache is a single shared, from-inception series across every caller; letting a later, narrower start_date shrink what gets fetched would silently truncate a cache other callers rely on being complete. start_date still filters what's returned in this case, just not what's fetched or persisted to disk.
• Checked every caller in the workspace before starting: the function's only real internal caller (update_all_daily_prices) never passes start_date, neither existing test passes it, and the other fetch_daily_ohlc in the tree (npf.ml.labels) is an unrelated function that correctly forwards its own start_date into cotdata.get_prices -- not this one.

Found while building a bounded, one-off Databento divergence-check script in a sibling repo (livebook), which had to write its own bounded fetch specifically because this function couldn't do it.

Test plan

• pytest tests/test_databento_provider.py -- 8 passed (2 existing + 6 new covering: cold-cache floor narrowing, GLBX-floor clamping still applies, return-value filtering, the None default is unchanged, and the warm-cache case -- pre-seeded 3-day cache, narrower start_date requested, asserts the fetch call still starts from last_date+1 while the returned frame excludes the pre-start row and the on-disk cache keeps the full series)
• pytest tests/ (full suite, COTDATA_STORE set) -- 86 passed
• Confirmed the 5 test_norgate_provider.py failures seen without COTDATA_STORE set are pre-existing and unrelated (reproduced identically with this change git stashed out)
• grep -rn "fetch_daily_ohlc(" across the workspace -- no current caller passes start_date to this function, so this is purely additive

🤖 Generated with Claude Code