InteractiveBrowserCredential: Add support for response_mode='form_post' to eliminate MSAL security warning

[zhuweid]

• Package Name: azure-identity
• Package Version: (latest)
• Operating System: All (not OS-specific)
• Python Version: Any

Describe the bug
When using InteractiveBrowserCredential, the following warning is emitted by MSAL:

response_mode='form_post' is recommended for better security. See https://www.rfc-editor.org/rfc/rfc9700.html#section-4.3.1

Currently, InteractiveBrowserCredential calls initiate_auth_code_flow without specifying response_mode="form_post". This defaults to a query-based flow, triggering the warning for less secure OAuth2 redirection. The local redirect handler (AuthCodeRedirectHandler) only implements a do_GET method and lacks do_POST support which is required to handle the callback when using form_post response mode.

To Reproduce
Steps to reproduce:

1. Use azure.identity.InteractiveBrowserCredential to authenticate in any Python app.
2. Observe the warning: response_mode='form_post' is recommended ... during interactive authentication.

Expected behavior

• InteractiveBrowserCredential should use response_mode="form_post" by default for better security (per MSAL and RFC 9700 recommendations).
• The redirect handler should be updated to handle POST (add a do_POST implementation that parses form body and stores query params in the same way as do_GET).
• After implementing this, the MSAL warning should no longer appear.

Screenshots

Additional context
See source: browser.py#L19

• Ref: oauth2.py MSAL warning
• This would align SDK with the latest OAuth best practices.

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