feat: native matrix URI pills for {user}/{room} + major rendering & codebase refactor

This change introduces native `matrix:` URI-based rendering for `{user}` and `{room}` placeholders,
replacing previous plaintext and matrix.to-based links. Users and rooms are now rendered as clickable
pills in supporting clients, with a clean display using display names and room names (no @/# prefixes).

Reporting, moderation, and auto-redaction messages have been updated to use the same rendering logic.
Inspect and event links now also use native `matrix:` URIs for direct in-client navigation.

Internally, URI generation and rendering logic have been unified via central helper functions,
ensuring consistent handling of user IDs, room IDs, aliases, and event IDs.

This commit also includes a broader refactor of the codebase:
- decomposed complex flows (e.g. join handling) into smaller helpers
- moved mutable class-level state to instance-level
- reduced duplicate API calls and redundant logic
- improved overall structure and maintainability

Test coverage has been extended for URI helpers and rendering logic to prevent regressions.

No breaking changes to existing template parameters like `{user_link}` or `{room_link}`.

tests/test_quality_regressions.py

@@ -0,0 +1,10 @@
+from community.bot import CommunityBot
+
+
+def test_matrix_uri_wrappers_delegate_to_canonical_helpers() -> None:
+    bot = CommunityBot.__new__(CommunityBot)
+    bot.config = {}
+    assert bot._matrix_user_uri("@alice:example.org") == "matrix:u/alice:example.org?action=chat"
+    assert bot._matrix_room_uri("!roomid:example.org") == "matrix:roomid/roomid:example.org"
+    assert bot._matrix_room_uri("!roomid:example.org", "#general:example.org") == "matrix:r/general:example.org"
+    assert bot._matrix_event_uri("!roomid:example.org", "$eventid") == "matrix:roomid/roomid:example.org/e/eventid"