feat: add support for custom server notifications

[bolinfest]

#556 introduced support for
custom client notifications, so this PR makes the complementary change, adding
support for custom server notifications.

MCP clients, particularly ones that offer "experimental" capabilities,
may wish to handle custom server notifications that are not part of the
standard MCP specification. This change replaces CustomClientNotification
with a more general CustomNotification type that can be used in both directions.

• CustomNotification can carry arbitrary methods/params while
  still preserving meta/extensions; wires it into the ServerNotification union
  and serde so params can be decoded with params_as
• allows client handlers to receive custom notifications via a new
  on_custom_notification hook
• adds integration coverage that sends a custom server notification end-to-end
  and asserts the client sees the method and payload

Test:

cargo test -p rmcp --features client test_custom_server_notification_reaches_client

[bolinfest]

@alexhancock hopefully this is relatively straightforward to review since it mirrors #556

[alexhancock]

LGTM in terms of approach!

But can we unify the CustomClientNotification and CustomServerNotification types into a single CustomNotification type that can be sent either direction? Would cut down on a lot of the duplicated logic we need to implement for each type

[bolinfest]

@alexhancock ah, so with the current implementation, we have these two things:

ts_union!(
    export type ClientNotification =
    | CancelledNotification
    | ProgressNotification
    | InitializedNotification
    | RootsListChangedNotification
    | CustomClientNotification;
);

ts_union!(
    export type ServerNotification =
    | CancelledNotification
    | ProgressNotification
    | LoggingMessageNotification
    | ResourceUpdatedNotification
    | ResourceListChangedNotification
    | ToolListChangedNotification
    | PromptListChangedNotification
    | CustomServerNotification;
);

I didn't realize it was possible to share types across the two (like CancelledNotification): will consolidate!

[bolinfest]

@alexhancock please re-review now that:

• I consolidated things into one CustomNotification type
• Updated the PR body to reflect this change
• Tightened up some of the tests

[alexhancock]

Yeah, looks great now. Thanks!