Logging: replace "Purge All Logs" with age-based delete control

[i-anubhav-anand]

What?

Closes #689

Replaces the single "Purge All Logs" button in the Manage Logs panel with an age-based delete control: a dropdown (Older than 30 days / 90 days / 1 year / All logs) followed by a "Delete" button.

Why?

The existing "Purge All Logs" button is too blunt — it forces users to delete their entire log history even when they only want to remove old entries. This change gives users meaningful control over how much history to keep, while still allowing a full purge via the "All logs" option.

How?

PHP:

• Added AI_Request_Log_Manager::delete_logs_older_than(int $days): int — delegates to the existing AI_Request_Log_Repository::cleanup_by_retention() for the batched SQL delete, or falls back to purge_all_logs() when $days === 0.
• Extended the DELETE /ai/v1/logs REST endpoint with an optional before_days integer param (minimum: 0). When omitted or 0, all logs are purged (backwards-compatible). Negative values are rejected with a 400.

React:

• Rewrote SettingsPanel to use a SelectControl (30 / 90 / 365 / 0 days) and a "Delete" button with a contextual confirmation message.
• Updated handleDeleteLogs in index.tsx to accept days: number, pass it as before_days in the DELETE request, and call fetchLogs() after deletion so the table immediately reflects actual remaining entries.

Tests:

• AI_Request_Log_ManagerTest: test_delete_logs_older_than_removes_only_old_entries, test_delete_logs_older_than_zero_purges_all
• AI_Request_Log_ControllerTest: test_purge_logs_with_before_days_removes_only_old_entries, test_purge_logs_rejects_negative_before_days

Use of AI Tools

AI assistance: Yes
Tool(s): Claude Code
Model(s): Claude Sonnet 4.6
Used for: Code scaffolding and test suggestions; all implementation details, logic, and tests were reviewed, tested locally, and edited by me.

Testing Instructions

1. Enable the AI Request Logging experiment under AI → Settings → Experiments.
2. Navigate to Tools → AI Request Logs.
3. Insert logs at different ages (or use the REST API / WP-CLI to backdate entries).
4. In the Manage Logs → Danger Zone panel, select an age threshold from the dropdown.
5. Click Delete → confirm → verify only logs older than the selected threshold are removed and the table immediately updates without a page refresh.
6. Select All logs and confirm → all entries are removed.
7. Verify the Delete button is disabled when there are no logs.

Screenshots or screencast

Before	After

Changelog Entry

Changed - Replace "Purge All Logs" button with an age-based delete control (30 days / 90 days / 1 year / All) in the Manage Logs panel.

[github-actions]

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Co-authored-by: i-anubhav-anand <anubhav24@git.wordpress.org>
Co-authored-by: dkotter <dkotter@git.wordpress.org>
Co-authored-by: itsgajendraSingh <gajendrasingh@git.wordpress.org>

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[dkotter]

E2E tests and ESlint are failing on this PR. Those will need cleaned up before we proceed with a review here.