Upgrade leptos_ssr_axum example to latest leptos_wasi and WASIp3

[codeitlikemiley]

This PR updates the leptos_ssr_axum example in examples/leptos_ssr_axum to align with the latest leptos_wasi changes, switching to the WASIp3 http trigger standard.

Changes Made

1. Updated Cargo.toml

• Set edition = "2024" and rust-version = "1.93.0".
• Set build = "build.rs".
• Added dependencies: bytes, futures, hydration_context, server_fn, wasip3, http, and http-body.
• Updated leptos_wasi to point to the official repository at revision 216acc484b0d6fe4b18876f1c96b68272498592b.
• Updated spin-sdk to version 6.0.0 with json feature.
• Configured client/server wasm-release profiles and targets (wasm32-wasip2).

2. Configured spin.toml

• Set executor = { type = "http" } for trigger.
• Changed component source to target target/wasm32-wasip2/release/leptos_ssr_axum.wasm instead of wasm32-wasip1.
• Configured build commands using WASI_RUNTIME=spin.

3. Added build.rs

• Created a standard build script that propagates WASI_RUNTIME environment variable into standard runtime_spin/runtime_wasmtime compiler flags.

4. Code Migration & Refactoring

• src/server.rs:
  • Migrated the incoming handler from the old WASIp1 Guest implementation to the latest wasip3::exports::http::handler::Guest pattern.
  • Initialized host async task scheduling using init_wasip3_spawner().
  • Converted WASIp3 Request to http::Request using wasip3::http_compat::http_from_wasi_request.
  • Configured Handler built natively using Handler::build(req).await and bound server functions.
  • Implemented serve_static_files to serve static assets from the root.
• src/app.rs:
  • Updated calls to spin_sdk::key_value::Store (e.g. open_default(), get_json(), set_json(), delete()) to be .awaited, as they are now asynchronous in Spin SDK v6.0.0.
  • Replaced direct spin_sdk::variables::get("agent_server_url") inside component functions with context-based routing (use_context::<AgentServerUrl>()) to facilitate isomorphic SSR/client operations.
  • Added warning-free silence blocks (let _ = id; let _ = call_id;) inside ToolCallView to eliminate compile-time warnings while preserving auto-generated component properties.