Harden workflow codegen module-name derivation

[sohumt123]

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(happy to share if useful for the Contributor role)

Description of changes

This is a build-tooling fix rather than a new/updated workflow. While rebuilding the crate I found two ways that innocently named spec files can silently drop a workflow or brick cargo build for everyone, so this PR hardens the module-name derivation in workflows/build.rs.

Problem

build.rs turns each spec's file name into a Rust module name and emits pub mod <name>;. Two edge cases produce broken generated code:

1. Module-name collisions (silent overwrite + build break). The name is derived from the basename via to_case(Case::Snake) with no uniqueness check. Two specs that differ only in separators/case — e.g. collide-test.yaml and collide_test.yaml — both normalize to collide_test. The generated .rs file for the first one is silently overwritten (one workflow is lost), and the module list emits a duplicate pub mod collide_test;:

   error[E0428]: the name `collide_test` is defined multiple times
      --> workflows/src/generated_workflows/mod.rs:185:1
   

   WalkDir recurses all of ../specs but only the basename is used, so this can even collide across different tool subdirectories.

2. Interior extension / stray dot. The extension is stripped with String::replace(".yaml", ""), which removes every occurrence, and the snake-case pass passes . through unchanged. So my.yaml.config.yaml becomes the invalid module my.config:

   error: expected one of `;` or `{`, found `.`
      --> workflows/src/generated_workflows/mod.rs:271:11
      |
   271| pub mod my.config;
      |           ^ expected one of `;` or `{`
   

Fix

• Factor the derivation into module_name(&str) -> String (new workflows/src/module_name.rs). It strips only the trailing .yaml/.yml, snake-cases the stem, replaces any remaining non-[A-Za-z0-9_] char with _, and prefixes _ if the result is empty or starts with a digit, so the output is always a valid identifier.

• build.rs now keeps a HashMap<module name, spec path> and, on collision, fails fast with a clear message naming both specs — which also prevents the silent overwrite:

  Error: workflow module name collision: ../specs/collide-test.yaml and ../specs/collide_test.yaml both normalize to module name `collide_test`; rename one of the spec files
  

• The helper is shared into build.rs via #[path] and compiled into the library under #[cfg(test)], so its unit tests actually run under cargo test (a build script has no test harness of its own). While touching that expression I also replaced the pre-existing .expect("OsStr should convert to str") with proper anyhow context.

Zero behavior change for the existing corpus: verified the new algorithm against all 332 current spec basenames — the generated module names are byte-for-byte identical, so downstream output is untouched.

Testing

New unit tests in module_name.rs (the crate previously had no Rust tests) pin the exact mappings, including the regressions above, plus a property check that every output is a valid identifier:

$ cargo test -p warp-workflows
running 5 tests
test module_name::tests::digit_leading_stem_is_prefixed ... ok
test module_name::tests::colliding_basenames_map_to_the_same_name ... ok
test module_name::tests::strips_only_the_trailing_extension ... ok
test module_name::tests::preserves_existing_snake_case_outputs ... ok
test module_name::tests::output_is_always_a_valid_identifier ... ok
test result: ok. 5 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out

Integration check with local-only fixtures (not committed):

# before this PR
$ printf 'name: Dot\ncommand: echo dot\n' > specs/my.yaml.config.yaml
$ rm -rf workflows/src/generated_workflows target && cargo build -p warp-workflows
error: expected one of `;` or `{`, found `.`   ->   pub mod my.config;

# after this PR
$ rm -rf workflows/src/generated_workflows target && cargo build -p warp-workflows
   Finished `dev` profile ...
$ grep my_yaml_config workflows/src/generated_workflows/mod.rs
pub mod my_yaml_config;

# colliding pair now fails fast in the build script (no E0428, nothing overwritten)
$ printf 'name: A\ncommand: echo a\n' > specs/collide-test.yaml
$ printf 'name: B\ncommand: echo b\n' > specs/collide_test.yaml
$ rm -rf workflows/src/generated_workflows target && cargo build -p warp-workflows
Error: workflow module name collision: ../specs/collide-test.yaml and ../specs/collide_test.yaml both normalize to module name `collide_test`; rename one of the spec files

After removing the fixtures, a full rebuild of the pristine 332-spec corpus passes and ls generated_workflows | sort diffs empty against a pre-change snapshot (zero module-name drift). cargo fmt --check, cargo clippy -- -A warnings, and cargo clippy --tests -- -A warnings all pass.