Add Nextcloud Talk tools

[bigcat88]

Summary

Adds 8 new MCP tools for Nextcloud Talk (Phase 2):

READ tools:

• list_conversations — list all conversations with unread counts, types, activity
• get_conversation — get details for a specific conversation by token
• get_messages — retrieve chat messages (newest first, configurable limit)
• get_participants — list participants with roles (owner/moderator/user/guest)

WRITE tools:

• send_message — send chat messages (Markdown, replies, UTF-8)
• create_conversation — create group or public conversations

DESTRUCTIVE tools:

• delete_message — delete a chat message (replaced with system message)
• leave_conversation — leave a conversation

All tools follow the established patterns: permission decorators, JSON return values, formatted output with only the most useful fields from the API response.

Tests: 44 new integration tests covering:

• All CRUD operations against live Nextcloud
• Message ordering, system messages, replies
• Conversation types (group vs public)
• Permission enforcement (READ/WRITE/DESTRUCTIVE boundaries)
• Error cases (nonexistent tokens, invalid room types)
• UTF-8 and Markdown content

Total test count: 153 (26 unit + 127 integration)

Verified against

• Nextcloud master (Talk 24.0.0-dev) — 153/153 passing
• Nextcloud stable33 (Talk 23.0.2) — 127/127 integration passing
• ruff check, ruff format, isort — clean
• pyright strict — 0 errors

Test plan

• All 44 Talk integration tests pass on NC master
• All 44 Talk integration tests pass on NC stable33
• Full test suite (153 tests) passes on NC master
• Full integration suite (127 tests) passes on NC stable33
• Lint and type checking clean
• Existing tests unbroken (test_server.py updated for new tool count)

[bigcat88]

Ready for review.

Summary: 8 new Talk tools with 44 integration tests, all passing on NC 31 + NC 32.

Tools added:

Tool	Permission	Description
list_conversations	READ	List all conversations with unread counts and activity
get_conversation	READ	Get details for a conversation by token
get_messages	READ	Retrieve chat messages (newest first, limit support)
get_participants	READ	List participants with roles
send_message	WRITE	Send messages (Markdown, replies, UTF-8)
create_conversation	WRITE	Create group or public conversations
delete_message	DESTRUCTIVE	Delete a chat message
leave_conversation	DESTRUCTIVE	Leave a conversation

CI also fixed: spreed + admin_notifications apps now installed in CI containers.

Total: 20 tools, 153 tests (26 unit + 127 integration).