DRAFT - feat: add multi-connection OAuth registry and flow APIs

[Lily Du (lilyydu)]

Summary

This PR adds the public OAuth flow/registry surface that lets apps register and
use multiple named OAuth connections instead of relying only on the app-level
default connection.

The shape mirrors the C# SDK’s OAuthFlow / AddOAuthFlow(...) model while
remaining additive for existing teams.py apps.

What changed

OAuth flow registry

• Added OAuthFlow, representing one named OAuth connection.
• Added OAuthFlowRegistry, a case-insensitive, insertion-ordered registry.
• Added App.add_oauth_flow(...) to register a connection and return its flow.
• Added App.get_oauth_flow(...) to retrieve a registered flow by connection name.
• Duplicate connection names raise ValueError case-insensitively.

Flow operations
Each OAuthFlow can now operate against its own connection:

github = app.add_oauth_flow("github")

await github.sign_in(ctx)
await github.sign_out(ctx)
token = await github.get_token(ctx)
signed_in = await github.is_signed_in(ctx)

OAuthFlow.sign_in(...) always forces the flow’s connection name, so callers cannot accidentally pass options for one connection through another flow.

ActivityContext OAuth helpers

• ctx.sign_out(connection_name=...) can now sign out a specific connection.
• ctx.get_user_token(connection_name=...) reads a token for a specific connection.
• Returns None for Token Service 404 / not signed in.
• Re-raises non-404 failures so service outages are not masked as logged-out users.
• ctx.get_token_status() retrieves status for all registered bot OAuth connections.

Events

• Added SignInFailureEvent.
• Sign-in failure now emits both:
  -> the existing ErrorEvent for backward compatibility;
  -> the new sign_in_failure event with connection name/code/message details.

Public exports

• Re-exported OAuthFlow and OAuthFlowRegistry from microsoft_teams.apps.