perf(importers): add defer_product_grading option + harden watson index-flush hook

[dogboat]

Summary

Two small, independent improvements for large imports/reimports, plus follow-up
hardening on the async watson index-flush hook. Both are backward compatible —
opt-in flag / no change to the default path.

defer_product_grading importer option

New ImporterOptions flag (default False), honored by both the importer and
reimporter. When enabled, product-grade recalculation is skipped during the run —
per dedupe batch, at end-of-process, and after closing old findings — so the caller
runs a single perform_product_grading() pass afterward instead.

On a large import/reimport the default path recalculates the product grade once per
dedupe batch (and again at the end), which is redundant; deferring collapses that to
one pass. The default (False) preserves existing behavior, so no current caller is
affected. The validator documents that it's all-or-nothing: with the flag set, the
caller owns the final grade.

Watson index-flush hook hardening

• suppress_intermediate_flush() — a thread-local context manager that turns the
  intermediate-flush hook into a no-op. watson.skip_index_update() only marks its
  context invalid on __exit__, so while a skip block is still accumulating, the flush
  hook can't tell it's a skip context and would drain it — dispatching async index
  tasks for objects meant to be discarded. Wrapping a skip_index_update() block in
  this makes the skip actually hold.
• Singleton guard — the hook now only intermediate-flushes the global singleton
  context. The async reindex task builds its own SearchContextManager and is itself
  the drain target; without the guard it would dispatch a clone of itself and loop.
  Ad-hoc contexts index their own batch on end() as before.

Minor

• Guards a reimporter debug log behind logger.isEnabledFor(DEBUG): it previously
  built a (id, hash_code) list over every original finding on each reimport,
  regardless of log level.

Tests

• test_importers_importer.py — importer honors the flag (per-batch dispatch off +
  no end grade), with a control proving the default path still grades.
• test_importers_closeold.py — the close-old grade is deferred, for both importer
  and reimporter.
• test_watson_intermediate_flush.py — ad-hoc contexts don't drain;
  suppress_intermediate_flush() no-ops and restores.

[sc-13510]

[valentijnscholten]

Funny how Opus generated the exact same code/naming on your PR as my PR.

In https://github.com/DefectDojo-Inc/dojo-pro/pull/1778 I have chosen a more elegant option. That one also adds a Celery context manager to make the async index batching work in celery tasks i.e. the background importer in pro.

Can I suggest to remove the watson parts of your PR?

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