Rust SDK: PR #1367 review follow-ups

[SteveSandersonMS]

Local review of #1367 (Rust SDK API cleanup) flagged 9 follow-ups: 3 fix-before-done items (broken doc build, an ordering hazard, and a silent-handler-drop footgun) plus 6 nice-to-haves. This PR addresses all 9 in a single pass so the API is in shape before the next round of dependents pin against it.

Fix-before-done

• cargo doc --no-deps is no longer broken on default features. tool.rs ToolHandler doc was using [Tool] and [define_tool] intra-doc links that fail rustdoc's broken-link lint when derive is off. Switched to fully-qualified paths for Tool and plain backticks for define_tool / schema_for. Same sweep applied to the README and a couple of types.rs items that linked to pub(crate) symbols.

• SessionConfig::to_wire / ResumeSessionConfig::to_wire are now consuming into_wire(self) -> Result<(Wire, SessionConfigRuntime), Error>. The new SessionConfigRuntime struct holds the trait-object handlers, tool-handler map, session-fs provider, and slash commands drained out of the config. This eliminates the deep Vec<Tool> / HashMap<String, Value> clone the old &self-based shape required, and makes the .take()-after-to_wire ordering hazard a compile-time impossibility. The duplicate-tool-handler check moved into into_wire as part of the drain.

• Serialize / Deserialize dropped from SessionConfig and ResumeSessionConfig. Every handler field had #[serde(skip)], so serde round-tripping silently dropped all runtime behavior. The wire payload is now produced exclusively via into_wire on the wire::Session{Create,Resume}Wire structs. Also dropped the now-redundant env_value_mode field on the configs (the wire struct hard-codes "direct"). Two *_serializes_bucket_b_fields integration tests moved into the in-crate unit-test module since the wire types are pub(crate).

Nice-to-haves

• Session rustdoc: get_messages -> get_events (the Phase A canonical name).
• Simplified the Client::start connection-token resolution block - dropped the shadow block, let _ = &mut options;, and the redundant retyped let.
• Tool::with_parameters now panics on non-object input via the existing tool_parameters helper instead of silently producing an empty parameter map. Matches the loud-on-misuse pattern of the rest of the tool module; in-tree call sites using json!({...}) are unaffected.
• README typo: second ToolHandler replaced with SystemMessageTransform in the handler-trait list.
• Trimmed the unreachable tool_name.is_empty() branch in the ExternalToolRequested dispatch path (the outer guard already short-circuits when the tools map lookup fails).
• Tool::handler moved from pub to pub(crate) and a Tool::handler() read-only accessor added. External callers can no longer overwrite an already-attached handler via direct field assignment; mutation goes through Tool::with_handler.

Public API impact

Three breaking changes worth calling out (all latent-bug or encapsulation fixes; SDK is pre-1.0):

• SessionConfig / ResumeSessionConfig lose Serialize + Deserialize (the path was always broken).
• Tool::with_parameters panics on non-object instead of silently dropping (real callers use json!({...}) and are unaffected).
• Tool::handler field becomes pub(crate); new Tool::handler() accessor is the read replacement.

Everything else is implementation-detail-only (to_wire was pub(crate), doc/typo fixes, dead-branch removal).

Validation

• cargo check --tests --all-features
• cargo doc --no-deps (default features) - confirms the fix-before-done #1 fix
• cargo doc --no-deps --all-features
• cargo test --lib --features test-support (128 passed)
• cargo clippy --all-targets --all-features -- -D warnings
• cargo fmt --check

Fixes: #1380

[Copilot]

Pull request overview

This PR applies the follow-ups from Rust SDK API cleanup (#1367) to harden the Rust SDK’s public surface and wire serialization path, while fixing rustdoc breakages on default features and removing a few remaining footguns.

Changes:

• Reworked session config wire-building to into_wire(self) -> Result<(Wire, SessionConfigRuntime), Error>, eliminating ordering hazards and deep cloning while centralizing duplicate tool-handler validation.
• Removed Serialize/Deserialize from SessionConfig and ResumeSessionConfig to prevent silent dropping of runtime handler fields; shifted wire-shape assertions into in-crate unit tests.
• Tightened tool API ergonomics and docs: Tool::with_parameters now panics on non-object schema input, Tool::handler is no longer publicly settable, and rustdoc/README links were fixed for default-feature builds.

Show a summary per file

File	Description
rust/tests/session_test.rs	Drops integration-test assertions that relied on serde wire-shape for configs; keeps token-redaction Debug coverage.
rust/src/types.rs	Implements into_wire + SessionConfigRuntime, removes serde derives from configs, enforces tool handler encapsulation, and updates tests accordingly.
rust/src/tool.rs	Fixes rustdoc links for default features and routes parameter-schema normalization through tool_parameters (panic-on-misuse).
rust/src/session.rs	Adopts into_wire runtime bundle, removes now-unreachable tool-name empty branch, and keeps handler extraction correct.
rust/src/lib.rs	Simplifies connection-token resolution logic in Client::start.
rust/README.md	Fixes handler-trait list typo and removes feature-gated rustdoc link to define_tool.

Copilot's findings

• Files reviewed: 6/6 changed files
• Comments generated: 0