fix(dhan): paginated statement API + history row shape

[SwathiMystery]

What

Fix the Dhan adapter to match the v2 API's actual behaviour. Historical-trades endpoint is GET /trades/{from}/{to}/{page} (page is a required 3rd path segment), not GET /trades/{from}/{to} as originally guessed. Also fix several smaller shape mismatches discovered during the first real sync.

Why

The first live khata sync --broker dhan call 4xx'd on the Statement API. Probing against a live account revealed the true endpoint shape plus three related quirks that would have silently corrupted data on ingest.

Changes

• client.py: get_trades_range now takes a page argument and maps to /trades/{from}/{to}/{page}.
• adapter.py: fetch_trades paginates each date chunk until the response is empty (safety-bounded at 100 pages).
• mapper.py:
  • Falls back to orderId when exchangeTradeId is "0" (history rows are aggregated per-order-per-day). Without this, UNIQUE(broker, broker_trade_id) would collapse every history row to a single insert.
  • Parses both "YYYY-MM-DD HH:MM:SS" (today's /trades) and "YYYY-MM-DDTHH:MM:SS" (history pages).
  • Handles literal "NA" in createTime / updateTime without crashing.
  • Strips trailing whitespace in underlying extraction so "NIFTY 21 APR 24300 PUT" → "NIFTY", not "NIFTY ".
• tests/fixtures/dhan/ + tests/test_dhan_mapper.py: 10 regression tests covering every fix above. Fixtures use synthetic IDs only (TEST_CLIENT_*, TEST_ORDER_*) — no real account data.

Verification

• 15/15 tests green locally (5 existing round-trip + 10 new mapper).
• ruff check + ruff format --check clean.
• Verified end-to-end against a live Dhan account: 103 executions over 30 days, 18 round-trip trades reconstructed.

Checklist

• Tests added (10 new cases, all green locally)
• uv run ruff check khata tests clean
• uv run ruff format khata tests applied
• Docs updated — public API unchanged; adapter internals only
• Fixture file + mapper tests included
• No schema change

Screenshots / output

$ uv run khata sync --broker dhan --since-days 30
→ authenticating with dhan…
→ fetching executions since 2026-03-21 …
  got 103 executions
  inserted 103 new rows
→ rebuilding round-trip trades…
  ✓ trades=18 (open=0) across 17 contracts