fix: retain conversation and tag mention identities on message reads

[amanjoshicodes]

Bug

message list and message get return "mentioned": {} for any mention that is not a person. An @everyone in a chat or channel, an @channel or @team, and a team tag mention are all stored by Graph and rendered by Teams, but the CLI's read-back shows them as unresolved.

Root cause: ChatMessageMentioned only modelled the user form of Graph's chatMessageMentionedIdentitySet, so serde dropped the conversation and tag identities on deserialisation. Same class as the subject drop fixed in #90: the send worked, the store was right, the read lied.

Before / after

Channel root carrying an @everyone, teams message get --team T --channel C ID -o json | jq '.data.mentions':

[{"id":0,"mentionText":"Everyone","mentioned":{}}]

with this change:

[{"id":0,"mentionText":"Everyone","mentioned":{"conversation":{"id":"19:...@thread.tacv2","displayName":"Everyone","conversationIdentityType":"channel"}}}]

The same message in a group chat via message list --chat goes from "mentioned": {} to a conversation with conversationIdentityType: "chat". A user mention on a neighbouring message is byte-identical before and after.

Change

• ChatMessageMentioned gains conversation (teamworkConversationIdentity: id, displayName, conversationIdentityType) and tag (teamworkTagIdentity: id, displayName), both optional and omitted from JSON when absent, so messages without them gain no null noise. It now derives Default; the two places that build a user mention use struct update syntax.
• application and device are deliberately left out (no delegated-flow use, nothing to verify against); an identity set holding only those still parses and prints as {} rather than failing the read. A test pins that.
• The send-side --mention shape is unchanged; the existing exact-shape serialisation test still passes untouched.
• CHANGELOG entry under Unreleased.

Tests

• chat_message_keeps_conversation_mentions_on_channels_and_chats (round trip for both channel and chat identity types)
• chat_message_keeps_tag_mentions
• chat_message_tolerates_unmodelled_mention_identities
• cargo fmt --all --check, cargo clippy --all-targets --all-features -- -D warnings, cargo test --all-targets: 348 passed
• Live: release build read back a real @everyone channel root and a real @everyone group-chat message with the shapes above; the brew 0.7.0 binary read both as {}.