feat(rust-server): refactor server module templates to avoid a monolithic run function.

[dsteeley]

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Summary by cubic

Refactors the rust-server generator to split the monolithic run() into per-operation handler functions, reducing compile-time memory and preventing rustc OOMs on large APIs. Behavior of generated servers and callbacks is unchanged.

• Refactors

  • Added server-operation-handler.mustache to generate async fn handle_<operation>() per operation.
  • Updated server-mod.mustache and client-callbacks.mustache to include handlers; server-operation.mustache now dispatches to them.
  • Kept request parsing, auth, validation, and response mapping logic intact within each handler.
  • Regenerated Rust server samples to reflect the new structure.

• Migration

  • No public API changes to generated code; existing implementations compile without changes.
  • No configuration or dependency updates required (hyper usage and behavior remain the same).

^{Written for commit 9bb4175. Summary will update on new commits.}