Fix the following issue:
The project compiler cache is used on every project load, but its current behavior is fragile: an empty contracts directory is treated as present, diagnostics crash when the directory is absent, filesystem traversal makes compiler input order vary, malformed cache files prevent a rebuild, and recompiling one source can leave contract types produced by an older version of that source in the manifest. These problems make a clean checkout noisy and can expose stale artifacts after normal edits or deletions.
Make the public ProjectManager behavior deterministic and self-healing while preserving the existing compiler/plugin interfaces. sources_missing must report true for a missing or empty contracts/ directory. sources must recursively discover files whose suffix has a registered compiler and return them in stable lexicographic order by their path relative to contracts/. extensions_with_missing_compilers must recursively collect unique unsupported file suffixes in the same stable order and return an empty list, rather than raising, when contracts/ is missing or empty.
load_contracts(use_cache=True) must treat a malformed local manifest cache (for example, invalid JSON) as a cache miss and rebuild from current sources. For a valid cache, unchanged sources must be reused without invoking their compiler again, while new or content-changed sources are compiled. Results belonging to a changed source must replace that source's prior contract types, and results belonging to deleted sources must disappear from the returned mapping and the persisted manifest. The persisted cache must describe the current source set so a subsequent load sees the same result. use_cache=False remains the explicit force mode and must recompile every current source, in deterministic order.