feat: add Reddit OAuth2 bundled provider

[ankitranjan7]

Motivation

This adds Reddit as a bundled OAuth provider. Reddit’s OAuth implementation differs from the default provider behavior in two important ways: token exchanges require HTTP Basic client authentication, and the authorization request needs a duration parameter when requesting long-lived access. Supporting Reddit cleanly meant making those provider-specific OAuth details configurable instead of hardcoding Reddit logic into the flow.

What Changed

• Added a bundled Reddit provider definition.
• Added provider-level support for token endpoint auth via either form-body credentials or HTTP Basic auth.
• Applied that token auth behavior across PKCE code exchange, device-code token polling, and refresh-token exchange.
• Added provider-level authorization URL parameters so providers like Reddit can request duration=permanent.
• Added regression coverage for provider authorization params and Basic-auth refresh behavior.
• Kept existing OAuth providers on the default form-body credential behavior.

[manojbajaj95]

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