[MM-63254] Add diagnostics data to the Support Packet

[hanzei]

Summary

When a Support Packet gets generated, the plugin now includes diagnostics data via a diagnostics.yaml file into the Packet.

Example content of com.mattermost.mscalendar/diagnostics.yaml:

version: 1.3.4
connected_user_count: 2
subscription_count: 5
is_oauth_configured: true

Requires mattermost/mattermost#28833 on the server side.

See https://mattermost.atlassian.net/wiki/spaces/CLOUD/pages/3083108354/Diagnostics+data+from+Core+Plugins+to+the+Support+Packet for more context.

Ticket Link

https://mattermost.atlassian.net/browse/MM-63254

PR Overview

This PR adds a diagnostics.yaml file to the Support Packet generated by the plugin, providing useful diagnostic information such as plugin version, connected user count, subscription count, and OAuth configuration status.

• Adds a new SupportPacket type and GenerateSupportData function in calendar/plugin/support_packet.go.
• Introduces GetSubscriptionCount and GetConnectedUserCount functions in the store layers and updates corresponding tests and mocks.
• Updates several test cases to use pointer-based user identifiers for consistency.

Reviewed Changes

File	Description
calendar/plugin/support_packet.go	Implements diagnostics data generation for the Support Packet.
calendar/store/subscription_store.go	Adds function to count subscriptions using paginated KVList calls.
calendar/store/user_store.go	Adds function to count connected users using paginated KVList calls.
calendar/utils/kvstore/*.go	Extends the KVStore interface with the List method and updates consumers.
Various test files (subscription_store_test.go, user_store_test.go, etc.)	Update tests to cover new counting functions and pointer-based user IDs.
calendar/config/config.go	Adds IsOAuthConfigured method to StoredConfig.
calendar/engine/*_test.go	Updates mock user creation to use pointer types.

Copilot reviewed 23 out of 23 changed files in this pull request and generated no comments.

Comments suppressed due to low confidence (1)

calendar/plugin/support_packet.go:50

• [nitpick] Clarify the usage of p.env.PluginVersion as the directory component in the filename; if this is not intended, consider using a dedicated constant or a more descriptive name for the diagnostics file path to improve clarity.

Filename: filepath.Join(p.env.PluginVersion, "diagnostics.yaml")

[wiggin77]

LGTM 👍 The KVStore ought to have some counting methods. Fetching all the keys just to count them seems inefficient. Some customers will require dozens of database hits just to count the keys.