fix(tags): keep user-set tags when creating a finding under product tag inheritance

[valentijnscholten]

Fixes #15092

Problem

When a product has tag inheritance enabled, creating a finding with its own tag via the UI ("add finding" flow) silently drops the user's tag — only the inherited product tags survive. Editing the finding afterwards and adding a tag works, which is what made the loss look specific to creation.

Reproduced exactly:

flow	result
create with pre-save tag (finding.tags = [...]; finding.save())	❌ ['producttag'] — user tag gone
add tag after save (edit flow)	✅ ['myrandomtag', 'producttag']
inheritance off	✅ user tag survives

Root cause — signal-handler ordering

The post_save dispatch order for Finding is:

1. imagekit → 2. watson → 3. dojo.tags.signals.inherit_tags_on_instance → 4. tagulous's flush (PostSaveHandler)

Tagulous holds tags assigned to an unsaved instance in an in-memory manager and only writes them to the through table from its own post_save handler, which is registered in tagulous' AppConfig.ready(). Because "dojo" precedes "tagulous" in INSTALLED_APPS, dojo's inheritance handler runs first: reading instance.tags on the freshly-saved row loads an empty, DB-backed manager and discards the pending pre-save tags before tagulous can persist them.

Fix

Before computing/merging inherited tags, flush the instance's own tag fields (tagulous manager.post_save_handler()) so the user's tags are committed first. This makes inheritance independent of signal-handler ordering — findings created with or without their own tags now end up correct.

• dojo/tags/signals.py — new _flush_pending_tag_fields() helper, called at the top of auto_inherit_product_tags().
• unittests/test_tag_inheritance.py — two regression tests (create-with-own-tag, create-without-tag). The first fails without the fix and passes with it.

Testing

• Reproduced the bug and validated the fix in a running stack (both cases correct).
• test_tag_inheritance (39), test_tags (26), test_tag_inheritance_perf (24) — all green.

[mtesauro]

Approved