fix(issues): validate add_issue_comment input modes

[SamMorrowDrums]

Summary

• model add_issue_comment as requiring at least one of body or reaction without making either top-level required
• require positive integer comment_id values to be paired with a valid reaction and reject comment bodies in existing-comment reaction calls
• preserve body-only comments, issue/pull request reactions, and combined comment-plus-issue-reaction behavior
• add focused schema and handler regression coverage and refresh the tool snapshot/docs

Validation

• UPDATE_TOOLSNAPS=true go test ./...
• script/generate-docs
• script/lint
• script/test

Fixes #3048

[Copilot]

Pull request overview

Fixes #3048 by aligning add_issue_comment schema validation with its supported input modes.

Changes:

• Makes body and reaction conditionally required.
• Validates comment IDs and reaction values.
• Adds regression tests and refreshes generated documentation.

Show a summary per file

File	Description
README.md	Documents comment_id as an integer.
pkg/github/issues.go	Adds schema and runtime input validation.
pkg/github/issues_test.go	Covers valid and invalid input modes.
pkg/github/__toolsnaps__/add_issue_comment.snap	Updates the tool schema snapshot.

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• Files reviewed: 4/4 changed files
• Comments generated: 0
• Review effort level: Balanced