arch(channels,gateway,a2a): extensible pub enums missing #[non_exhaustive]

[bug-ops]

Description

Several extensible pub enums across four crates are missing `#[non_exhaustive]`, which forces downstream crates to add wildcard match arms on any upgrade if a new variant is ever added.

Enum	Crate	File
`AnyChannel`	`zeph-channels`	`any.rs:54`
`TelegramApiError`	`zeph-channels`	`telegram_api_ext.rs:143`
`ChatMemberStatus`	`zeph-channels`	`telegram_api_ext.rs:95`
`ReadLineResult`	`zeph-channels`	`line_editor.rs:32`
`SlashCategory`	`zeph-commands`	`lib.rs:110`
`GatewayError`	`zeph-gateway`	`error.rs:10`
`A2aError`	`zeph-a2a`	`error.rs:16`
`ProcessorEvent`	`zeph-a2a`	`server/state.rs:42`
`CancelError`	`zeph-a2a`	`server/state.rs:247`
`IbctError`	`zeph-a2a`	`ibct.rs:48`

`TaskState`, `Role`, `Part`, and `TaskEvent` in `zeph-a2a/types.rs` already have `#[non_exhaustive]` — the above are the remaining gaps.

Expected Behavior

All extensible pub enums have `#[non_exhaustive]` so that adding a variant in a patch release does not break downstream match expressions.

Actual Behavior

Missing attribute — downstream match expressions will require updates on any variant addition.

Environment

• Version: HEAD 05d2cc8
• Detected during CI-887 architecture audit

Fix

Add `#[non_exhaustive]` to each enum listed above. Part of the broader `non_exhaustive` sweep tracked in #4513, #4514, #4515, #4517, #4520, #4527.