Add on-call pages list

[platinummonkey]

What does this PR do?

Adds pup on-call pages list so users can list on-call page records from the CLI. The command wraps the currently usable unstable on-call pages endpoint while the stable v2 collection endpoint remains unavailable for listing.

Motivation

Issue #642 documents that pup on-call pages only supported create and get, leaving users without a programmable CLI path to inspect page volume by team or responder. This addresses the CLI-side gap in this repository while keeping the backend stable-endpoint request tracked separately.

Changes

• Added an on-call pages list subcommand with --team, --responder, and --page-size options.
• Fetches pages from /api/unstable/on-call/pages, applies team filtering server-side with filter=team:<handle>, and applies responder filtering client-side against relationships.responders.
• Validates page size locally as 1..=1000 to match the unstable endpoint cap.
• Added focused tests for query construction, invalid page-size handling, responder matching/filtering, and read-only command classification.
• Updated README.md and docs/COMMANDS.md to include on-call page listing.

Testing

• cargo +stable fmt --check
• git diff --check
• cargo +stable test on_call_pages --locked was attempted but blocked before compilation because Cargo could not fetch the pinned datadog-api-client git dependency in this sandbox; the GitHub fetch returned 403.

Additional Notes

This intentionally does not claim stable pagination or time-window support. The unstable endpoint does not provide working pagination, server-side responder filtering, or in-query time filtering, so those remain backend/API limitations.

Checklist

• The code change follows the project conventions (see CONTRIBUTING.md)
• Tests have been added/updated (if applicable)
• Documentation has been updated (if applicable)
• All CI checks pass
• Code coverage is maintained or improved

Related Issues

Related to #642.

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Bits Code status: ✅ Done

CI Auto-fix: Disabled | Enable

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