detect: fail thread init on keyword ctx error

[ssam18]

Contribution style:

• I have read the contributing guide lines at
  https://docs.suricata.io/en/latest/devguide/contributing/contribution-process.html

Our Contribution agreements:

• I have signed the Open Information Security Foundation contribution agreement at
  https://suricata.io/about/contribution-agreement/ (note: this is only required once)

Changes (if applicable):

• I have updated the User Guide (in doc/userguide/) to reflect the changes made
• I have updated the JSON schema (in etc/schema.json) to reflect all logging changes
  (including schema descriptions)
• I have created a ticket at
  https://redmine.openinfosecfoundation.org/projects/suricata/issues

Link to ticket: https://redmine.openinfosecfoundation.org/issues/8237

Describe changes:

• ThreadCtxDoInit called DetectEngineThreadCtxInitKeywords but ignored its return value. When a per-thread keyword init fails (for example DetectFilemagicThreadInit returning NULL), the keyword context array was left partially initialized and the detect thread kept running, giving indeterminate results.
• Propagate the failure from DetectEngineThreadCtxInitKeywords so ThreadCtxDoInit returns TM_ECODE_FAILED. The existing callers already deinit and fail thread init on that, so the thread now aborts cleanly instead of running degraded.
• Added a unit test that registers a keyword whose per-thread init returns NULL and verifies DetectEngineThreadCtxInit reports failure. Confirmed the test fails without the fix and passes with it, and the full DetectEngine unit test group still passes.

[ssam18]

Superseded by #15589, which keeps this keyword-init fix (Ticket #8237) and also carries the cbindgen/bytemath build fix needed to get CI green. Closing in favor of that one.