fix(auth): surface 'No token' dispatch failure as AuthError::NoToken

[sandikodev]

Issue

Closes #3173

Problem

When a session expires mid-chat, the AWS SDK returns a DispatchFailure with "No token" in the error chain. This was converted to ChatError::Client and displayed as an opaque multi-line error:

Amazon Q is having trouble responding right now:
   0: Failed to send the request: dispatch failure (other): No token
   1: dispatch failure (other): No token
   2: dispatch failure
   3: other
   4: No token

The handler for ChatError::Auth(AuthError::NoToken) already exists and displays a clear, actionable message — but it was never reached because the error was wrapped as ChatError::Client.

Fix

Add is_no_token_error() which walks the error source chain to detect the "No token" string. In From<ApiClientError> for ChatError, check for this condition and convert to ChatError::Auth(AuthError::NoToken) so the existing handler is triggered, showing:

Authentication Error

Your login session has expired. Please log in again using:

    q login

Testing

# Run the targeted tests
cargo test -p chat_cli is_no_token_error

# Run the full test suite to check for regressions
cargo test -p chat_cli

Four tests cover all cases:

1. detects_auth_no_token — ApiClientError::AuthError(AuthError::NoToken) is detected
2. ignores_unrelated_errors — unrelated errors (e.g. DefaultModelNotFound) are not detected
3. from_api_client_error_converts_no_token_to_auth_error — From conversion produces ChatError::Auth(AuthError::NoToken)
4. from_api_client_error_keeps_other_errors_as_client — other errors remain as ChatError::Client

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[sandikodev]

Update (amended commit): Tests were added after initial submission covering four cases:

1. ApiClientError::AuthError(AuthError::NoToken) is detected by is_no_token_error
2. Unrelated errors (e.g. DefaultModelNotFound) are not detected
3. From<ApiClientError> converts a No-token error to ChatError::Auth(AuthError::NoToken)
4. From<ApiClientError> keeps other errors as ChatError::Client

This ensures the error chain walking logic is correct and the From conversion behaves as expected in both the bug case and the normal case.