fix(rg): search piped stdin when no paths are given

[mohan-garimella]

Summary

When rg receives piped stdin with no explicit path arguments, it ignores stdin and defaults to recursively searching the current working directory (.). This is inconsistent with real ripgrep and with how grep already handles stdin in just-bash.

Reproduction

# rg ignores stdin, searches cwd instead
echo "hello world" | rg "hello"
# Expected: hello world
# Actual: recursively searches . and returns all file matches containing "hello"

# grep handles stdin correctly
echo "hello world" | grep "hello"
# Output: hello world ✅

Fix

After building the regex, check for non-empty stdin before defaulting to .. When stdin has content and no paths are given, searchContent is called directly on the stdin text with all applicable options (invert, count, context, quiet, -l, etc.).

Also fixes rg.utf8-stdin.test.ts which was previously passing by accident — it tested cat /in.txt | rg '한글' but rg was actually finding the match by searching the filesystem (it found in.txt), not by reading stdin.

Tests

• 7 new stdin-specific tests covering: basic piped search, no-match exit code, case-insensitive, inverted match, count mode, explicit-path-overrides-stdin, and multibyte UTF-8
• All 515 existing rg tests pass (82 skipped)
• All 205 existing grep tests pass (8 skipped)

Fixes #280

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[mohan-garimella]

Addressed in 15f5d15 — added a stdinConsumedByPatternFile guard that skips the stdin search path when -f - was used. Also added a test for echo "hello" | rg -f - /data.txt to ensure patterns from stdin are used against the explicit file, not searched as content.

[cramforce]

Please sign commits

[mohan-garimella]

@cramforce Done, Please review.