Fixes BEP path handling for rust analyzer specs

[csmulhern]

The rewrite of the Rust Analyzer auto-discovery pipeline (see #4075) introduced a bug for users who use --experimental_convenience_symlinks=ignore (i.e. have no bazel-out symlink in their workspace).

parse_output_group_paths documentation suggests that output paths are always absolute

rules_rust/tools/rust_analyzer/bep.rs

Lines 107 to 110 in dedf053

	/// Read a BEP JSONL file and return every path that appears in the named
	/// output group of any completed target or aspect, deduplicated. Paths are
	/// absolute on the local filesystem.
	pub fn parse_output_group_paths(

but that is not true for BEP events that trigger the fallback reconstruction path

rules_rust/tools/rust_analyzer/bep.rs

Lines 241 to 243 in dedf053

	// Fallback: reconstruct from pathPrefix + name. Bazel uses this form
	// when the file lives in bazel-out and the absolute URI isn't reported.
	if let Some(name) = &file.name {

This PR detects those cases and anchors those paths to the execution root, making them also absolute.

[csmulhern]

cc @UebelAndre

[UebelAndre]

Nice! Thank you!