V2: add compatible base providers for completions messages and responses #34999

Context

We want Completions-compatible, Messages-compatible, and Responses-compatible as reusable base provider families. Named providers should compose these bases with endpoint/auth/defaults/options/model quirks rather than cloning protocol logic.

Scope

• Clarify or introduce base entrypoints for Completions-compatible, Messages-compatible, and Responses-compatible protocols.
• Define how named providers register endpoint/auth/defaults/options/model-specific quirks.
• Keep provider-specific behavior out of generic protocol code unless it is genuinely protocol-wide.

Acceptance Criteria

• There is a clear base for /chat/completions compatible providers.
• There is a planned or implemented base for /responses compatible providers.
• There is a planned or implemented base for Anthropic Messages-compatible providers.
• Named providers compose bases instead of duplicating protocol implementations.