fix(grep): keep FilePath scope in regex/literal fallback (#756)

[gustav-fff]

Closes #756

Root cause

Two layers. The AI-mode parser splits on whitespace, so regex fragments ^/\* and |^\s*/\* (both contain /+*) are misparsed as Glob constraints, leaving only ---------- as grep text. Those bogus globs match nothing, so the constrained search is empty and the literal/regex fallback at crates/fff-core/src/grep/grep.rs:232 rebuilt the query with constraints: Vec::new(), discarding the user's FilePath("scope/target.css") scope. In regex mode the raw query is then re-run whole and the top-level | branch ^\s*/\* ---------- matches /* ---------- in every file.

Fix

crates/fff-core/src/grep/grep.rs: the fallback now preserves FilePath constraints (only the swallowed operator/glob tokens are dropped), so a pinned path can't be escaped. Regression test added in grep_tests.rs.

Steps to reproduce

On pre-fix origin/main:

mkdir /tmp/fff-756 && cd /tmp/fff-756
npm init -y && npm install @ff-labs/fff-node@0.10.3

Save repro.mjs from the issue, then:

node repro.mjs ./out.log

Expected: actualPaths limited to ["scope/target.css"], filteredFileCount/totalFilesSearched = 1.
Actual (pre-fix):

{"actualPaths":["outside.css","scope/target.css"],"totalMatched":2,"filteredFileCount":2,"totalFilesSearched":2}

How verified

Added e2e regression test reproducing the exact query through the core pipeline (AiGrepConfig + regex mode):

cargo test -p fff-search grep
# test result: ok. 6 passed; 0 failed  (incl. regex_fallback_keeps_file_path_scope_issue_756)
cargo test -p fff-search --test grep_integration
# test result: ok. 68 passed; 0 failed
cargo clippy -p fff-search   # clean

Pre-fix the new test yields ["outside.css","scope/target.css"]; post-fix ["scope/target.css"].

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