[tracking] Go Anthropic plugin parity

[cabljac]

Tracking issue for Go Anthropic plugin parity with JS. The JS plugin has citations, the beta API surface, and structured thinking config; Go has none of the three. Each gap is its own item:

• feat(go/plugins/anthropic): add document and web-search citation support #5459 document and web-search citation support
• feat(go/plugins/anthropic): support beta API surface and web-search server-tool results #5463 beta API surface and web-search server-tool results
• feat(go/plugins/anthropic): add structured extended-thinking config (budget/adaptive/display) #5464 structured extended-thinking config (budget/adaptive/display)

Related recent work: Claude model registration parity landed via #5494 (Go) and #5496 (Py).

[a2105z]

Hi @cabljac , let me take a look at this and see what I can do.