fix(wasm): address #1086 review followups -- description hint and coercion safety

[zmanian]

Summary

Followup fixes from the review of #1086 (tool_info schema discovery), which was merged before review comments were addressed.

• Replace fragile description string mutation with composition at display time. The old append_schema_hint_if_permissive() / strip_schema_hint() mutated the stored description string using contains / find / truncate, coupling presentation concerns to stored state. Now the raw description stays clean and the tool_info hint is composed in the Tool::schema() override only when the advertised schema is permissive. The hint also now includes the tool name and include_schema: true for better LLM guidance.

• Make effective_for_coercion use load-time schema instead of re-calling WASM export mid-execution. The old code called tool_iface.call_schema() on the already-running WASM instance between store setup and call_execute(), risking state contamination from mutable linear memory. Now falls back to the PreparedModule.schema extracted at load time.

Test plan

• cargo check clean
• cargo clippy --all --benches --tests --examples --all-features zero warnings
• cargo fmt clean
• test_advertised_schema_stays_permissive_until_sidecar_override updated and passing
• test_coerce_noop_with_permissive_schema passing
• test_tool_info_* tests passing

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[gemini-code-assist]

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