Expose semantic testTags on chat-ui affordances

[phwizard]

Summary

Adds Compose `Modifier.testTag(...)` markers to the chat-ui affordances that both Compose UI tests and Maestro E2E flows need to resolve at runtime. Today tests have to fall back to `contentDescription` / text matching, which breaks under localization, theming, and minor copy edits.

Tags added

Component	Tag	Element
`ChatInput`	`chat_input`	`OutlinedTextField` for user text
`ChatInput`	`chat_send_button`	FAB (active) AND disabled IconButton (empty field) — same tag on both states so tests can resolve the send affordance without knowing whether text is present
`ChatInput`	`chat_attach_button`	Attach IconButton (hidden in edit mode)
`MessageBubble`	`chat_message_image`	The `MediaMessage` subtree for media bubbles
`RoomListView`	`rooms_list`	The `LazyColumn` rendering rooms
`RoomListView`	`room_row`	Each clickable room `Surface`
`RoomListView`	`rooms_search_input`	The `SearchBar` `OutlinedTextField`
`RoomListView`	`create_room_button`	The "+" Add IconButton in the top app bar

Pattern

Each component declares its tag values in a small companion `*TestTags` object at the end of the file:

```kotlin
object ChatInputTestTags {
const val INPUT_FIELD = "chat_input"
const val SEND_BUTTON = "chat_send_button"
const val ATTACH_BUTTON = "chat_attach_button"
}
```

Source consumers import constants rather than re-typing magic strings, so:

• Tags stay discoverable next to the composable they describe.
• Renaming a tag is a one-place change.
• Documents in one place that tests + Maestro flows depend on these values; don't change them casually.

Behavior

No behavior change. `testTag` is a `SemanticsModifier` — purely additive metadata only observable from accessibility / test frameworks. Render tree, layout, click handling unchanged.

Companion PRs

• `ethora-sdk-android` PR #3 — existing Compose UI tests can update to `onNodeWithTag(...)` instead of `onNodeWithContentDescription(...)` once this lands.
• `ethora-sample-android` PR #2 — the Maestro flows in `.maestro/flows/` already reference these exact id values (`id: "chat_input"`, `id: "chat_send_button"`, etc.) but as `optional: true` so they no-op today; once this lands those become required assertions and the flows actually exercise their full intent.

Test plan

• `./gradlew :chat-ui:assembleDebug` succeeds.
• No regressions in existing `./gradlew :ethora-component:test` suite.
• Visual / behavior of `ChatInput`, `MessageBubble`, `RoomListView` unchanged on the sample app (manual smoke).
• After the companion test PR is rebased on this, `./gradlew :chat-ui:connectedDebugAndroidTest` resolves the new `onNodeWithTag(...)` finders correctly.