Add p3 readonly truncation regression test

[angelnereira]

Summary

• Add a p3 filesystem regression test for opening a read-only preopened file with TRUNCATE.
• Reuse the p3 test harness with a configurable WasiCtxBuilder so the test can install a read-only file preopen.
• Verify that the open fails with NotPermitted and that the file contents remain unchanged.

Context

This extends coverage for the permission invariant behind GHSA-2r75-cxrj-cmph / CVE-2026-47261 to the p3 filesystem path. The underlying fix is already present in the shared filesystem implementation; this PR adds p3-specific regression coverage so the invariant stays protected as p3 evolves.

Testing

• cargo fmt --all -- --check
• cargo test -p wasmtime-wasi --features p3 --test all p3_file_truncation_readonly
• git diff --check