[Docs] API Reference section in README is missing return types and error variants for MockEnvBuilder methods

[ekelemepraise-code]

Summary

The API Reference section in README.md lists method signatures for MockEnvBuilder but omits:

• Return types (every method just says -> Self)
• Error conditions (what panics, what validates)
• Available feature flags that gate certain methods

What's Missing

Method	Missing Info
.with_wasm(wasm: &[u8])	No mention of what happens with invalid WASM
.with_protocol_version()	No list of valid versions
.capture_logs()	No mention of how to retrieve captured logs
.track_costs()	No mention of feature flag requirement

Proposed Work

1. Expand each builder method with a "Panics" / "Errors" sub-section
2. Add feature flag annotations (e.g., [requires: feature = "snapshots"])
3. Cross-link to examples in examples/ for each method
4. Add a "Common Mistakes" callout box

Acceptance Criteria

• Every public MockEnvBuilder method documented with panics and errors
• Feature flags clearly annotated
• Prose reviewed and consistent