fix(read): prioritize known text extensions

[original4422]

Summary

• Treat known text extensions as authoritative before content sniffing, preventing embedded magic-byte literals from turning source files into image/PDF payloads.
• Preserve infer detection for unknown and extensionless files and preserve the existing extension fallback.
• Exercise the real ForgeFsRead path and MIME compatibility matrix with same-file regression tests.

Root cause

infer::get scans the entire supplied buffer and recognizes the %PDF sequence at byte offset 449. Limiting the buffer to the issue's suggested 1 KiB would still include that sequence and therefore would not fix this reproduction. The minimal reliable fix is to trust the existing known-text extension allowlist before content sniffing.

Validation

• cargo test -p forge_services tool_services::fs_read::tests -- --nocapture — 28 passed
• cargo test -p forge_services — 216 passed; doc tests passed
• cargo check -p forge_services — passed
• cargo clippy -p forge_services --all-targets --all-features -- -D warnings — passed
• cargo fmt -p forge_services -- --check — passed
• git diff --check — passed

cargo insta test was unavailable because the local cargo-insta command is not installed, so the complete crate suite was run directly with cargo test. A workspace-wide clippy attempt stopped while building the untouched forge_repo crate because local protoc is absent; CI installs protoc, and the touched crate's all-target/all-feature clippy is green.

Fixes #3812

Co-Authored-By: ForgeCode noreply@forgecode.dev

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