Decide on input handling for generate command

**_{} Use frontend language when defaulting IR input path**

When no `--input` is provided, the generate command builds the IR path using the *target* language (`resolve_compile_output(..., &target_lang, ...)`). If a user compiles from a different frontend language (e.g., `gleam`) and then generates to another target (e.g., `scala`), the IR is stored under `out//compile/gleam`, but generate will look under `compile/scala` and fail with “IR input path does not exist”. This breaks the default workflow whenever frontend and target differ; the default should be derived from the frontend language in config (or the compile output used by the last compile step), not the backend target.

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_Originally posted by @chatgpt-codex-connector[bot] in https://github.com/finos/morphir-rust/pull/36#discussion_r2724203967_