Add unified support conversations endpoints

[jkmassel]

Description

Wires /wpcom/v2/mobile-support/unified-conversations into WpComApiClient. This is the newer unified support endpoint that supersedes /mobile-support/conversations; the response shape is different enough that reusing the existing SupportConversation types isn't a good fit.

Changes

• New module wp_com::unified_conversations with response/param types:
  • UnifiedConversationSummary (list item — adds can_accept_reply: bool vs. the older summary)
  • UnifiedConversation + UnifiedMessage (flat message / author_role / author_name shape — not the older nested SupportMessageAuthor enum)
  • UnifiedAttachment with metadata: HashMap<String, JsonValue> — reference-link attachments carry a float score, and the existing AttachmentMetadataValue is Eq-bound so can't hold f64. Reusing the crate-wide JsonValue avoided either breaking Eq on the existing type or inventing a parallel value enum.
  • ReplyToUnifiedConversationParams { message: String } — JSON body, no attachments (the current API doesn't accept them on reply)
  • Reuses ConversationId from support_tickets (the IDs are in the same namespace)
• New UnifiedConversationsRequest endpoint enum under WpComNamespace::V2:
  • GET /mobile-support/unified-conversations
  • GET /mobile-support/unified-conversations/<conversation_id>
  • POST /mobile-support/unified-conversations/<conversation_id>
• Wired through wp_com/mod.rs, wp_com/endpoint.rs, and wp_com/client.rs (builder + executor + api_client_generate_endpoint_impl!)

Test plan

• cargo build -p wp_api
• cargo test --lib -p wp_api unified_conversations — 6 tests pass
  • List fixture deserializes to 70 items; can_accept_reply / status spot-checked
  • Single-conversation fixture deserializes with 10 messages; bot-response attachments verified to contain a JsonValue::Float score
  • Reply-response fixture (same shape as single GET) deserializes
  • Endpoint URL tests for list / get one / reply
• cargo fmt --all -- --check
• cargo clippy -p wp_api --tests --all-features -- -D warnings
• End-to-end hit against public-api.wordpress.com from the example app (follow-up)

[wpmobilebot]

XCFramework Build

This PR's XCFramework is available for testing. Add to your Package.swift:

.package(url: "https://github.com/automattic/wordpress-rs", branch: "pr-build/1288")

_{Built from bc3f97d}