refactor(anthropic): GA structured output support

[sixlive]

Description

Anthropic's structured outputs feature has moved from beta to GA. Native structured output is now the default strategy for Anthropic — no beta header (structured-outputs-2025-11-13) required.

Changes

Strategy selection (Structured.php):

• Native output format is now the default (previously required beta header)
• use_tool_calling provider option is the explicit opt-out for older models
• Removed hasNativeStructuredOutputSupport() beta header check
• Removed JsonModeStructuredStrategy import (no longer reachable)

Tests:

• Flattened describe('native structured outputs') block — no longer gated behind beta config
• Removed two citation + structured output tests (this combination is incompatible at the Anthropic API level)
• Updated payload test to expect output_config instead of JSON-mode prompt injection
• Fixed message index in thinking test (no extra JSON-mode message appended)

Docs:

• Native structured output documented as default (no beta setup needed)
• Strict tool use no longer references beta header
• Citations section clarifies incompatibility with structured output
• Config comment updated to reference remaining beta features (output-128k, code-execution)

Breaking Changes

• Structured output now uses native mode by default instead of prompt-based JSON mode
• Citations can no longer be used with structured output (previously worked via JSON mode workaround, but this is unsupported by the Anthropic API)
• Users who had the beta header configured for structured outputs can remove it