Fix lastSeen schema validation

[cursor]

Allow lastSeen in IssueSchema to be nullable to correctly parse Sentry API responses.

The Sentry API can return lastSeen: null for issues under certain conditions, but the existing IssueSchema defined lastSeen as a non-nullable string, leading to ZodError during parsing. This change updates the schema to reflect the API's behavior.

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[cursor]

Cursor Agent can help with this pull request. Just @cursor in comments and I'll start working on changes in this branch.
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