Emit dedicated error message for Conda environment.yml files (#12669)

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## Summary
Fixes #12606.

Two options considered, thanks to @zanieb's guidance are:
1. Special-casing on parse error and encountering the `environment.yml`
filename, possibly at `RequirementsTxt::parse`
2. Adding a new `RequirementsSource::EnvironmentYml` variant and
erroring on `RequirementSpecification::from_source`

I went with the latter for the following reasons:
- This edge case is explicitly modelled within the type system. However,
it changes the semantics of `RequirementsSource` to also model
_unsupported_ sources.
- (**Separation of concerns**) The special-casing would occur in the
`uv-requirements-txt` crate, which seems to be relatively deep in the
guts of the codebase. In my opinion, maintainers working in
`uv-requirements-txt` would reasonably assume the input file to be a
`requirements.txt` file, instead of having to be concerned with it being
another file format (`environment.yml`, `pyproject.toml`, etc.)

<!-- What's the purpose of the change? What does it do, and why? -->

## Test Plan
Manually tested as follows:
```sh
>>> cargo run -- pip install -r environment.yml
error: Conda environment file `environment.yml` is not supported

>>> cargo run -- add -r environment.yml
error: Conda environment file `environment.yml` is not supported
``` 

If you can point me to the appropriate test module, I can write up tests
for these to use `insta`.

<!-- How was it tested? -->

crates/uv-requirements/src/sources.rs

@@ -23,6 +23,8 @@ pub enum RequirementsSource {
     SetupCfg(PathBuf),
     /// Dependencies were provided via a path to a source tree (e.g., `pip install .`).
     SourceTree(PathBuf),
+    /// Dependencies were provided via an unsupported Conda `environment.yml` file (e.g., `pip install -r environment.yml`).
+    EnvironmentYml(PathBuf),
 }
 
 impl RequirementsSource {
@@ -35,6 +37,8 @@ impl RequirementsSource {
             Self::SetupPy(path)
         } else if path.ends_with("setup.cfg") {
             Self::SetupCfg(path)
+        } else if path.ends_with("environment.yml") {
+            Self::EnvironmentYml(path)
         } else {
             Self::RequirementsTxt(path)
         }
@@ -217,6 +221,7 @@ impl std::fmt::Display for RequirementsSource {
             | Self::PyprojectToml(path)
             | Self::SetupPy(path)
             | Self::SetupCfg(path)
+            | Self::EnvironmentYml(path)
             | Self::SourceTree(path) => {
                 write!(f, "{}", path.simplified_display())
             }

crates/uv-requirements/src/specification.rs

@@ -212,6 +212,12 @@ impl RequirementsSpecification {
                     ..Self::default()
                 }
             }
+            RequirementsSource::EnvironmentYml(path) => {
+                return Err(anyhow::anyhow!(
+                    "Conda environment files (i.e. `{}`) are not supported",
+                    path.user_display()
+                ))
+            }
         })
     }
 

crates/uv/tests/it/edit.rs

@@ -3662,6 +3662,41 @@ fn add_error() -> Result<()> {
     Ok(())
 }
 
+/// Emit dedicated error message when adding Conda `environment.yml`
+#[test]
+fn add_environment_yml_error() -> Result<()> {
+    let context = TestContext::new("3.12");
+
+    let pyproject_toml = context.temp_dir.child("pyproject.toml");
+    pyproject_toml.write_str(indoc! {r#"
+        [project]
+        name = "project"
+        version = "0.1.0"
+        requires-python = ">=3.12"
+        dependencies = []
+    "#})?;
+
+    let environment_yml = context.temp_dir.child("environment.yml");
+    environment_yml.write_str(indoc! {r"
+        name: test-env
+        channels:
+          - conda-forge
+        dependencies:
+          - python>=3.12
+    "})?;
+
+    uv_snapshot!(context.filters(), context.add().arg("-r").arg("environment.yml"), @r"
+    success: false
+    exit_code: 2
+    ----- stdout -----
+
+    ----- stderr -----
+    error: Conda environment files (i.e. `environment.yml`) are not supported
+    ");
+
+    Ok(())
+}
+
 /// Set a lower bound when adding unconstrained dependencies.
 #[test]
 fn add_lower_bound() -> Result<()> {

crates/uv/tests/it/pip_install.rs

@@ -8049,6 +8049,32 @@ fn install_incompatible_python_version_interpreter_broken_in_path() -> Result<()
     Ok(())
 }
 
+/// Emit dedicated error message when installing Conda `environment.yml`
+#[test]
+fn install_unsupported_environment_yml() -> Result<()> {
+    let context = TestContext::new("3.12");
+
+    let environment_yml = context.temp_dir.child("environment.yml");
+    environment_yml.write_str(indoc! {r"
+        name: test-env
+        channels:
+          - conda-forge
+        dependencies:
+          - python>=3.12
+    "})?;
+
+    uv_snapshot!(context.filters(), context.pip_install().arg("-r").arg("environment.yml"), @r"
+    success: false
+    exit_code: 2
+    ----- stdout -----
+
+    ----- stderr -----
+    error: Conda environment files (i.e. `environment.yml`) are not supported
+    ");
+
+    Ok(())
+}
+
 /// Include a `build_constraints.txt` file with an incompatible constraint.
 #[test]
 fn incompatible_build_constraint() -> Result<()> {