baml_language: add PPIR stream diagnostics and centralize virtual file handling

[miguelcsx]

This improves the stream-types pipeline by validating @stream.* usage earlier in PPIR and by centralizing virtual file classification in SourceFile.

Mainly inspired by: 06910c5

Before this change:

• virtual file detection was spread across crates with hardcoded prefixes
• invalid/conflicting stream attributes were often only handled indirectly or later
• diagnostics for malformed stream attributes could be noisier or less precise
• PPIR name collection could behave inconsistently in duplicate-name scenarios

Now we:

• validate stream attributes directly in PPIR
• report clearer diagnostics for unknown, invalid, and conflicting stream attributes
• use a single canonical API for builtin/generated file classification
• make temp-test projects safer and more isolated

Impact

• earlier, clearer feedback for invalid @stream.* usage
• fewer hardcoded virtual-path checks across the compiler
• more robust diagnostic spans and less noisy reporting on parse-broken files
• more stable PPIR behavior when duplicate names exist
• safer, deterministic integration tests

What changed

• added ppir_stream_diagnostics and integrated it into project diagnostics
• added ConflictingStreamAttributes diagnostic plumbing and error code
• centralized builtin/generated file prefix logic in baml_base::SourceFile
• updated call sites to use canonical builtin path helpers
• improved PPIR validation span selection and parse-error gating
• preserved first-seen PPIR name metadata instead of overwriting on duplicates
• added PPIR diagnostics integration tests and switched them to real temp dirs

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[miguelcsx]

Hi @sxlijin, I was reviewing your commit on PPIR and had a question: should @stream.type(...) support complex types, or only simple names/types? I’d prefer not to complicate things unnecessarily.