fix(incremental): bound boundary-dedup state growth (#4030)

[AndreaBozzo]

Fixes #4030.

Problem

When many records share the boundary (maximum) cursor value, dlt stores one deduplication hash per record in unique_hashes and re-writes the full list into _dlt_pipeline_state on every run, with no upper bound. A backfill that stamps every row with the same timestamp, or a low-resolution cursor (a date without a time), produces hundreds of thousands of hashes — tens of MB written to the state table on each run, growing the destination's pipeline-state table at run_count * state_size.

Why not just drop the hashes

The hashes are load-bearing. The closed range start re-fetches boundary rows on the next run, and unique_hashes is how dlt filters the ones already emitted. Dropping them (e.g. the issue's suggestion to skip them when primary_key is set) re-appends every boundary row on the next run — i.e. it reintroduces duplicate data. I confirmed this with a repro: a 3-run pipeline stays at exactly 1000 rows because of the hashes; without them it would grow to 2000/3000.

So this PR keeps the dedup semantics intact and instead makes the cost visible and boundable:

• Opt-in hard cap. New Incremental.duplicate_cursor_error_threshold (ClassVar, None/disabled by default) raises the new terminal IncrementalCursorThresholdExceeded once the boundary-hash count crosses it, so state can't silently balloon. Fails fast on the batch that crosses the bound.
• Actionable warning. The existing duplicate_cursor_warning_threshold warning now names the concrete mitigations: use a higher-resolution cursor, or switch to merge_key + range_start="open" to disable boundary deduplication, or set the error threshold to fail instead of accumulating.
• Docs. New "Growing pipeline state from boundary deduplication" section in the incremental troubleshooting page.

Default behavior is unchanged — the cap is off unless explicitly set.

Tests

• test_duplicate_cursor_error_threshold: default keeps all hashes and never raises; with the threshold set, crossing it raises IncrementalCursorThresholdExceeded (terminal, wrapped in PipelineStepFailed).
• test_duplicate_cursor_warning_threshold: the warning fires once and includes the range_start='open' / duplicate_cursor_error_threshold guidance.