browser_session() and BrowserClient.start() do not support proxyConfiguration

### Description

The [Browser Proxy](https://docs.aws.amazon.com/bedrock-agentcore/latest/devguide/browser-proxies.html) feature launched GA on February 10, 2026, and the underlying `StartBrowserSession` Boto3 API fully supports the `proxyConfiguration` parameter. However, the `bedrock-agentcore` SDK (v1.3.1) does not expose this parameter in `browser_session()` or `BrowserClient.start()`.

Customers who want to use proxy configuration through the high-level SDK are forced to drop down to raw Boto3 calls, bypassing the SDK's session lifecycle management, WebSocket header generation, and live view URL generation.

### Expected behavior

```python
from bedrock_agentcore.tools.browser_client import browser_session

proxy_config = {
"proxies": [
{
"externalProxy": {
"server": "proxy.example.com",
"port": 8080,
"domainPatterns": [".example.com", ".internal.corp"],
"credentials": {
"basicAuth": {
"secretArn": "arn:aws:secretsmanager:us-east-1:123456789012:secret:proxy-creds"
}
}
}
}
],
"bypass": {
"domainPatterns": [".amazonaws.com", "169.254.169.254"]
}
}

with browser_session("us-east-1", proxy_configuration=proxy_config) as client:
ws_url, headers = client.generate_ws_headers()
# ... use proxied session
```

### Current behavior

`browser_session()` accepts only `region`, `viewport`, and `identifier`. There is no way to pass proxy configuration through the SDK.

### Proposed changes

1. Add `proxy_configuration: Optional[Dict] = None` parameter to `BrowserClient.start()`, passing it as `proxyConfiguration` to the Boto3 `start_browser_session` call
2. Add `proxy_configuration: Optional[Dict] = None` parameter to `browser_session()`, forwarding it to `BrowserClient.start()`
3. Add TypedDict definitions for the proxy configuration structure (improves developer experience with IDE autocomplete)
4. Bump minimum `boto3`/`botocore` to `>=1.42.54` (the version that added `proxyConfiguration` to the `bedrock-agentcore` service model)

### SDK version

`bedrock-agentcore` v1.3.1