feat: Add Explicit Termination Mechanism for Multi-Agent Workflows (Similar to AutoGen "TERMINATE")

## Is your feature request related to a problem? Please describe.

In multi-agent workflows using ADK, agents can sometimes fall into infinite or
long-running loops during collaborative tasks. This happens especially when
orchestration depends only on message passing, with no built-in signal to
indicate that the task is completed.

Other multi-agent frameworks — for example, Microsoft's AutoGen — include
explicit termination mechanisms (e.g., `TextMentionTermination`,
`StopMessageTermination`, or custom termination callbacks) that help teams of
agents know when to gracefully stop.

In ADK today, there is no simple built-in mechanism for agents to signal "task
completed," which can lead to runaway conversations, wasted tokens, and stalled
agent workflows.

## Describe the solution you'd like

ADK should support a standardized termination mechanism that allows:

- **Agents to explicitly signal termination**, for example by producing a
special message type (like `StopMessage` in AutoGen) that ends the
conversation.
- **Teams to define termination conditions**, such as detecting a specific
keyword (`"TERMINATE"`), max message counts, function call results, token
usage, or any other customizable logic.
- **External termination control**, so applications integrating ADK can stop a
run programmatically (e.g., a UI "Stop" button).
- **Composable termination conditions**, allowing multiple rules combined with
`AND`/`OR` logic (similar to AutoGen's
`MaxMessageTermination | TextMentionTermination`).

This would give ADK parity with AutoGen's termination system and prevent
infinite loops in multi-agent workflows.

## Describe alternatives you've considered

- **Custom stop messages in agent prompts** — unreliable; agents may not follow
instructions consistently.
- **Max iterations as a safeguard** — prevents infinite loops but doesn't allow
agents to end a task early when done.
- **External timeout logic** — helps with long runs but doesn't let agents
indicate intentional completion.

None of these provide the simple, robust, and explicit termination controls that
AutoGen offers.

## Additional context

AutoGen's termination system includes the following primitives, all of which
this proposal aims to replicate in ADK:

| Condition | Description |
|---|---|
| `TextMentionTermination` | Stop when a keyword appears in content |
| `MaxMessageTermination` | Stop after N events |
| `TokenUsageTermination` | Stop when token budget is exceeded |
| `TimeoutTermination` | Stop after a wall-clock duration |
| `FunctionCallTermination` | Stop when a specific function is called |
| `ExternalTermination` | Stop on external async signal |
| `StopMessageTermination` | Stop on a special stop message type |

## Cross-repository alignment

This feature is being implemented in parallel across both ADK implementations to
keep them aligned:

- **adk-js** — Feature request: google/adk-js#29
- **adk-js** — Implementation PR (under review): google/adk-js#193
- **adk-python** — Implementation PR: https://github.com/google/adk-python/pull/5213

The goal is for both `adk-python` and `adk-js` to ship the same termination
primitives with a consistent API surface, so developers can reason about
termination the same way regardless of which SDK they use.