Add AI triage eligibility module with profile support

What does this MR do and why?

Adds Vulnerabilities::AiTriage::Eligibility module that makes an attached triage_and_remediation profile a second source of truth for whether an AI vulnerability triage flow may run for a project. It sits alongside the existing duo_sast_fp_detection_enabled, duo_secret_detection_fp_enabled and duo_sast_vr_workflow_enabled project settings, and follows a similar shape as DependencyManagement::SecurityUpdate::Eligibility.

It exposes enabled?, severity_allowed? and auto_run? over three trigger types: sast_false_positive, secret_detection_false_positive and sast_vulnerability_resolution. The profile_for and configuration_for lookups behind them are memoized per project and trigger in Gitlab::SafeRequestStore.

The call sites OR the profile check into their existing eligibility predicates and gate the automatic enqueue on auto_run?. It replaces a hardcoded high_or_critical_severity? check with severity_allowed?. It also applies the existing with_projects scope to preload the project that the per-vulnerability checks now read.

Enablement and severity live in the predicates the workers re-check, so the automatic and bulk paths agree. run_mode sits only on the automatic callbacks, which the bulk path bypasses, so manual means "only when a user asks" rather than disabled.

Every predicate is a no-op where no profile drives the trigger. severity_allowed? returns the hardcoded high/critical gate when no profile threshold governs, and auto_run? returns true, since the settings path has no manual mode. secret_detection_false_positive has no severity_level in its trigger schema, so it always takes that fallback.

Enablement is an OR, configuration is profile-first:

Project setting	Profile attached	Enabled?	Severity	Run mode
✅	❌	✅	hardcoded high or critical	always automatic
❌	✅	✅	profile severity_level	profile run_mode
✅	✅	✅	profile	profile
❌	❌	❌	-	-

Changelog: added
EE: true

Related issue

[Backend] Introduce a new Eligibility module fo... (#627964) • Gal Katz

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