fix: preserve multipart upload filenames across clients

[adamcavendish]

Summary

IR:

• Add request-body-specific multipart input planning for object-shaped multipart/form-data
• Keep canonical schema models schema-faithful, including non-multipart format: binary
• Track per-part default filename fallbacks from multipart wire field names

Clients:

• Generate multipart request input models such as {Operation}MultipartRequestBody
• Emit upload wrappers only when multipart request input fields need them
• Derive upload filenames from caller-provided wrapper/File values across TypeScript, Go, Python, Java, Kotlin, and Rust
• Keep text and JSON multipart parts on their existing code paths

Tests:

• Regenerate multipart-capable goldens across all registered generators
• Pin dynamic filename behavior in cross-client smoke coverage, including Rust reqwest/ureq/aioduct

Validation

• cargo test --test multipart_runtime_smoke
• just golden-check
• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets --all-features -- -D warnings
• git diff --check

Generated-client build checks:

• npx --yes -p typescript -c 'just golden-build-all' passed TypeScript tsc 69/69, TypeScript vp 5/5, and Go 57/57 before Rust registry download failures interrupted the full run.
• just golden-python::build passed Python-httpx 57/57 and Python-requests 57/57.
• just golden-java::build passed Java 58/58.
• just golden-kotlin::build passed Kotlin 58/58.
• Rust generated build sections were intermittently blocked by crates.io connection resets/timeouts in unrelated fixtures, but multipart fixtures passed for reqwest, ureq, and aioduct during the retry runs.