feat(php-shim): give ported exceptions PHP's class hierarchy

Ported exceptions were flat structs reached with `downcast_ref`, so
Composer's `catch (\RuntimeException $e)` only matched the exact leaf
type and `get_class($e)` had nothing to report. Each exception now
embeds an instance of the class it extends and travels inside an
`AnyThrowable`; `Catch::catch`/`catch_mut` walk that chain, and
`PhpClass::php_class_name` yields the PHP FQCN.

Dropping the `std::error::Error` impls from the exception types leaves
`AnyThrowable` as the only route into an `anyhow::Error`, so the walk
cannot be bypassed. A `no_exception_downcast` linter catches the
`downcast::<X>()` calls that would now silently answer `None`.

Three sites change behavior as a result: the `TransportException`
exit-code override reaches `MaxFileSizeExceededException`, the
`catch (\LogicException)` in findSimilar() reaches its subclasses, and
rendered exception titles carry the real class name rather than a
guess. `get_class_err()` is no longer a `todo!()`, which re-enables
FilesystemRepositoryTest::testCorruptedRepositoryFile.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

1 files changed, 9 insertions, 15 deletions

diff --git a/crates/shirabe/src/downloader/fossil_downloader.rs b/crates/shirabe/src/downloader/fossil_downloader.rs
index 72409a1f..86a44f0b 100644
--- a/crates/shirabe/src/downloader/fossil_downloader.rs
+++ b/crates/shirabe/src/downloader/fossil_downloader.rs
@@ -47,14 +47,11 @@ impl FossilDownloader {
             .execute(&command, output, cwd.as_deref())?
             != 0
         {
-            return Err(RuntimeException {
-                message: format!(
-                    "Failed to execute {}\n\n{}",
-                    command.join(" "),
-                    self.inner.process.borrow().get_error_output()
-                ),
-                code: 0,
-            }
+            return Err(RuntimeException::new(format!(
+                "Failed to execute {}\n\n{}",
+                command.join(" "),
+                self.inner.process.borrow().get_error_output()
+            ))
             .into());
         }
         Ok(())
@@ -168,13 +165,10 @@ impl VcsDownloader for FossilDownloader {
         ));
 
         if !self.has_metadata_repository(path) {
-            return Err(RuntimeException {
-                message: format!(
-                    "The .fslckout file is missing from {}, see https://getcomposer.org/commit-deps for more information",
-                    path
-                ),
-                code: 0,
-            }.into());
+            return Err(RuntimeException::new(format!(
+                "The .fslckout file is missing from {}, see https://getcomposer.org/commit-deps for more information",
+                path
+            )).into());
         }
 
         let real_path = shirabe_php_shim::realpath(path);