[PM-38966] feat: Add fill assist opt-in setting and privacy gate

[aj-rosado]

🎟️ Tracking

https://bitwarden.atlassian.net/browse/PM-38966

📔 Objective

Settings UI

• Adds a "Turn on fill assist" toggle to Settings → Autofill → Additional options, visible only when the FillAssistTargetingRules feature flag is enabled
• Toggle is off by default (explicit opt-in)
• Includes an info icon linking to https://bitwarden.com/help/fill-assist/

Privacy gate

• FillAssistManagerImpl.syncIfNecessary() now exits early when the user has not opted in — no background CDN requests while the feature is disabled
• Toggling the setting on triggers an immediate syncIfNecessary() call so rules are available without waiting for the next server-config emission

Autofill integration

• AutofillParserImpl applies fill-assist rules only when both the feature flag and isFillAssistEnabled are true
• When rules match the current host, site-specific field targeting replaces heuristics entirely; unmatched nodes are excluded (no heuristic fallback)

Data layer

• New isFillAssistEnabled per-user preference in SettingsDiskSource / SettingsRepository (keyed with appendIdentifier(userId), stored unencrypted)
• Full network stack: FillAssistService, manifest + forms JSON parsing, 6-hour re-fetch throttle with CID-based deduplication

CSS selector matching

• Parses tag#id, [attr='value'], CSS descendant selectors, and Shadow DOM (>>>) notation
• Descendant-selector splitting is bracket-aware so attribute values containing spaces (e.g. [placeholder='Email address']) are preserved
• HtmlInfo.matchesSelectorClause() extracted to HtmlInfoExtensions following the existing android.util.Pair isolation pattern

Tests

• FillAssistManagerTest: new case — sync does nothing when isFillAssistEnabled = false
• AutoFillViewModelTest: toggle-on persists setting and calls syncIfNecessary(); toggle-off persists setting and skips sync; info-click emits NavigateToFillAssistHelp
• AutoFillScreenTest: switch hidden when flag off, switch visible when flag on, toggle sends FillAssistToggleClick, NavigateToFillAssistHelp calls intentManager.launchUri
• FillAssistViewNodeExtensionsTest: all previously commented-out tests now pass via mockkStatic(HtmlInfo::matchesSelectorClause)

📸 Screenshots