Fix a bug where no warning is output when parsing of workspace settings fails. (#4014)

## Summary

See #4013

`uv pip ...` command loads workspace settings from pyproject.toml and
uv.toml.

Although a warning is implemented to output a warning when parsing
fails, it is not actually output.


https://github.com/astral-sh/uv/blob/main/crates/uv-workspace/src/workspace.rs#L38-L61

The reason is that the flag to display warnings is enabled after loading
the workspace settings.

This PR turns on the warning output flag before loading the workspace.

## Test Plan

pyproject.toml for test

```toml
[project]
name = "sample"
version = "0.0.0"
dependencies = ["ruff"]

[tool.uv.pip]
# originally string type.
index-url = 1
```

command output (before modification)
```bash
uv pip compile pyproject.toml 

Resolved 1 package in 383ms
# This file was autogenerated by uv via the following command:
#    uv pip compile pyproject.toml
ruff==0.4.7
    # via sample (pyproject.toml)
```

command output (after modification)

```bash
uv pip compile pyproject.toml

warning: Failed to parse `pyproject.toml`: TOML parse error at line 7, column 13
  |
7 | index-url = true
  |             ^^^^
invalid type: boolean `true`, expected a string

Resolved 1 package in 107ms
# This file was autogenerated by uv via the following command:
#    uv pip compile pyproject.toml
ruff==0.4.7
    # via sample (pyproject.toml)
```

crates/uv-warnings/src/lib.rs

@@ -17,6 +17,11 @@ pub fn enable() {
     ENABLED.store(true, std::sync::atomic::Ordering::SeqCst);
 }
 
+/// Disable user-facing warnings.
+pub fn disable() {
+    ENABLED.store(false, std::sync::atomic::Ordering::SeqCst);
+}
+
 /// Warn a user, if warnings are enabled.
 #[macro_export]
 macro_rules! warn_user {

crates/uv/src/main.rs

@@ -107,6 +107,11 @@ async fn run() -> Result<ExitStatus> {
         }
     };
 
+    // enable flag to pick up warnings generated by workspace loading.
+    if !cli.global_args.quiet {
+        uv_warnings::enable();
+    }
+
     // Load the workspace settings, prioritizing (in order):
     // 1. The configuration file specified on the command-line.
     // 2. The configuration file in the current directory.
@@ -148,7 +153,9 @@ async fn run() -> Result<ExitStatus> {
     };
 
     // Configure the `warn!` macros, which control user-facing warnings in the CLI.
-    if !globals.quiet {
+    if globals.quiet {
+        uv_warnings::disable();
+    } else {
         uv_warnings::enable();
     }
 

crates/uv/tests/pip_compile.rs

@@ -3048,7 +3048,6 @@ fn override_multi_dependency() -> Result<()> {
 }
 
 /// Check how invalid `tool.uv.override-dependencies` is handled in `pyproject.toml`.
-// TODO(konsti): We should show a warnings here or better fail parsing.
 #[test]
 fn override_dependency_from_workspace_invalid_syntax() -> Result<()> {
     let context = TestContext::new("3.12");
@@ -3095,6 +3094,14 @@ fn override_dependency_from_workspace_invalid_syntax() -> Result<()> {
         # via flask
 
     ----- stderr -----
+    warning: Failed to parse `pyproject.toml`: TOML parse error at line 9, column 29
+      |
+    9 |     override-dependencies = [
+      |                             ^
+    no such comparison operator "=", must be one of ~= == != <= >= < > ===
+    werkzeug=2.3.0
+            ^^^^^^
+
     Resolved 7 packages in [TIME]
     "###
     );

crates/uv/tests/pip_install.rs

@@ -142,6 +142,12 @@ fn invalid_pyproject_toml_syntax() -> Result<()> {
     ----- stdout -----
 
     ----- stderr -----
+    warning: Failed to parse `pyproject.toml`: TOML parse error at line 1, column 5
+      |
+    1 | 123 - 456
+      |     ^
+    expected `.`, `=`
+
     error: Failed to parse: `pyproject.toml`
       Caused by: TOML parse error at line 1, column 5
       |