[codex] Add plain text fallback for unsupported extensions

[buger]

Fixes #580.

Summary

• Added plain-text fallback handling for unsupported extensions in symbols and query instead of failing or silently skipping useful text.
• Added standard text treatment for .1, .5, .txt, .conf, .tex, .sh, and .json.
• Added --strict to keep the old skip/error behavior and repeatable --text-extension EXT to force additional extensions through text search.
• Added a shared text-search file guard used by regular search, filename matches, query, and symbol extraction. It keeps hard-denied binary/archive/media/font/database/object extensions out of text search, rejects NUL-containing files, rejects non-regular files, skips symlinks/junctions, and enforces the existing 1 MiB read limit consistently.
• Documented the new flags and behavior in the CLI docs.

Test and Stability Work

• Added focused tests for symbols/query fallback, strict mode, standard text extensions, explicit language behavior, custom text extensions, hard-denied extension override attempts, oversized text files, and file-list filtering.
• Replaced slow nested cargo run test harness usage with the compiled test binary path and removed stale target/debug/probe assumptions.
• Fixed pre-existing test failures uncovered by the full suite: stale outline assertions, JSON/XML output expectations, filename required-term handling, exact/excluded query ranking and filename false positives, deterministic fixture setup, LSP runtime handling, LSP comprehensive isolation/bootstrap, Python symbol fallback, C# tree-sitter ABI pin, and stale doctests.

Validation

• cargo fmt --check
• cargo check --tests
• cargo test
• cargo test --doc
• Focused guard/fallback checks: cargo test file_guard --lib, cargo test symbols_ --lib, cargo test --test query_command_tests query_ -- --nocapture, cargo test file_list_ --lib
• Focused regression checks: cargo test --test integration_tests -- --nocapture, cargo test --test filename_search_tests -- --nocapture, cargo test --test required_terms_filename_tests -- --nocapture, cargo test --test json_format_tests -- --nocapture
• Stress loop: cargo test --test quoted_term_with_negative_keyword_tests -- --nocapture x5
• Anti-pattern scan for nested cargo run/hardcoded target binaries in tests returned no matches.
• Leftover LSP/server process scan returned no matches.