feat: Add dismissal_restriction to PullRequestRuleParameters

[benlangfeld]

A ruleset's pull_request rule can restrict who may dismiss a review, the same way a branch protection rule's dismissal_restrictions does. PullRequestRuleParameters stops short of it, so a ruleset that has one round-trips through this client without it: unmarshalling drops the restriction, and marshalling the result back sends a rule that no longer has one.

The REST API accepts and returns it as pull_request.parameters.dismissal_restriction, and describes it as repository-rule-params-dismissal-restriction:

{
  "title": "DismissalRestriction",
  "description": "Specify people, teams, or apps allowed to dismiss pull request reviews.",
  "properties": {
    "allowed_actors": { "type": "array", "items": { "$ref": "#/components/schemas/repository-rule-params-actor" } },
    "enabled": { "type": "boolean" }
  },
  "required": ["enabled"]
}

A live ruleset carries it whether or not it is in use — this is a repository ruleset created without one:

{"type": "pull_request", "parameters": {"dismissal_restriction": {"allowed_actors": [], "enabled": false}, ...}}

enabled is required by the schema, so it is a plain bool; allowed_actors is optional and omitted when empty.

The actor type is its own enum rather than a reuse of BypassActorType, because GitHub documents a different set for this field: User, Team, IntegrationInstallation, RepositoryRole. An app is IntegrationInstallation here and Integration in a bypass actor, so sharing the type would let a value that is valid in one place be written into the other.

Accessors are regenerated. Two round-trip cases are added to TestRepositoryRule: one restricting dismissal to a team and the admin repository role, and one enabling the restriction with no allowed actors, which is how "administrators only" is expressed.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.59%. Comparing base (48d0a66) to head (aee1387).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4599   +/-   ##
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  Coverage   98.59%   98.59%           
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  Files         197      197           
  Lines       18326    18326           
=======================================
  Hits        18068    18068           
  Misses        258      258           

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