Add policy badge for block_branch_modification in branch rules UI

What does this MR do and why?

Extends the PolicyBadge and DisabledByPolicyPopover pattern to cover block_branch_modification in the branch rules UI. Previously, only prevent_pushing_and_force_pushing showed policy indicators (badge + lock/warning icon) in branch rules. The block_branch_modification setting only disabled the Delete Rule button. Now users see clear visual feedback across all affected settings sections when a security policy blocks branch modification.

This follows the exact same pattern established in !221912 (merged) for prevent_pushing_and_force_pushing.

Changes

• EE list page query: Add modificationBlockedByPolicy and warnModificationBlockedByPolicy fields
• EE branch_rule.vue: Extend showPolicyBadge to include modification blocking; add isEnforcedByAnyPolicy computed to aggregate multiple policy types for the badge variant
• CE index.vue (detail page): Pass modificationBlockedByPolicy/warnModificationBlockedByPolicy to "Allowed to merge", "Allowed to push", and Force push sections
• CE branch_rule.vue: Add isEnforcedByAnyPolicy stub; update PolicyBadge/PopOver bindings
• Tests: Comprehensive coverage for enforced mode, warn mode, and combined policies

No backend changes needed — all GraphQL fields already exist.

References

• Related to #575666 (closed)
• Follows pattern from !221912 (merged) (prevent_pushing_and_force_pushing policy badge)

Screenshots or screen recordings

Before	After
No policy indicator for block_branch_modification on list or detail pages

How to set up and validate locally

1. Go to Project > Secure > Policies > New approval policy
2. Create a policy with block_branch_modification: true enabled:
   • First test in warn mode, then test in enforced mode
3. Go to Settings > Repository > Branch Rules
4. Verify the branch rules list page:
   • A PolicyBadge should appear next to the branch name (e.g., "Enforced by policy" or "Policy warn mode")
5. Click View details on a branch rule
6. Verify the branch rule detail page:
   • "Allowed to merge" section: Edit button should be disabled (enforced) with lock icon popover, or show warning icon (warn mode)
   • "Allowed to push and merge" section: Same behavior
   • Force push toggle: Should be disabled (enforced) with lock icon popover
   • Delete rule button: Should already be disabled (existing behavior, unchanged)
7. Verify that without any blocking policy, all sections behave normally (no badges, no popovers)

MR acceptance checklist

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