fix(core): respect off severity in intermediate rulesets

[jurijat]

Fixes #2889.

Checklist

• Tests added / updated
• Docs added / updated

Does this PR introduce a breaking change?

• Yes
• No

If indicated yes above, please describe the breaking change(s).

Before the fix:

const parent = {
  extends: [[oas, 'off']],
  rules: { 'some-rule': true }  // Re-enables rule
};

const child = {
  extends: [[parent, 'off']]  // BUG: 'some-rule' stayed enabled
};

After the fix: Same code, but now some-rule is correctly disabled

Additional context

• Fixed bug where extends: [[ruleset, "off"]] did not disable rules that were explicitly re-enabled in intermediate rulesets
• Added severity filtering for inherited rules when explicit severity modifier is used
• Documentation was not edited, as it was already correct.

Test plan

• Added 6 new test cases covering:
  • Deep nesting (4 levels with 'off')
  • Mixed severity modifiers followed by 'off'
  • Object rule modifications followed by 'off'
  • Non-recommended rules enabled then disabled by 'off'
  • Child re-enabling rules after extending with 'off'
  • Original bug scenario with intermediate rule enable