🐛 fix: byte-boundary panic (#148) + rmcp allowed_hosts env vars (#149)

[flupkede]

Two unrelated community-reported issues bundled in one PR per maintainer direction.

#148 — UTF-8 panic in search snippet output

Reported by @tony-nexartis: codesearch search panicked with byte index 100 is not a char boundary when a result snippet contained multi-byte UTF-8 characters (box-drawing separators in comment art, CJK, emoji) at the truncation point.

Root cause: &snippet[..100] byte-sliced a UTF-8 string. Byte offset 100 falling inside a multi-byte character is a panic.

Originally flagged in PR #152 review as "out-of-scope, deferred"; reported as #148. (My sanitize_for_terminal from #152 preserves multi-byte chars, which paradoxically made this panic deterministically reproducible for box-drawing content.)

Fix: str::floor_char_boundary(100) (stabilized in Rust 1.82; we're on 1.95) finds the largest char boundary ≤ 100 bytes, then slice. 1-line change.

Test: test_byte_truncation_preserves_char_boundary constructs a 120-byte string of 40 × U+2500 (─) — byte 100 lands inside char #33 (bytes 99–102), so the pre-fix code genuinely panics on this fixture.

#149 — rmcp allowed_hosts env var overrides

Reported by @stdweird: containerised deployments of codesearch serve fail because rmcp ≥ 1.4.0's DNS-rebinding defence (GHSA-89vp-x53w-74fx, CVE-2026-42559) defaults StreamableHttpServerConfig::allowed_hosts to loopback-only ["localhost", "127.0.0.1", "::1"]. The container's Host header (its hostname) is rejected with WARN ... rejected request with disallowed Host header.

Fix: expose two env vars, both read once at serve startup:

Env var	Effect
CODESEARCH_ALLOWED_HOSTS=host[,host:port,...]	Comma-separated list replaces the rmcp default allowlist. Whitespace-trimmed, empties dropped.
CODESEARCH_DISABLE_HOST_VALIDATION=1|true	Disables Host validation entirely (disable_allowed_hosts() → empty allowlist → rmcp allows all hosts). Dangerous — only safe behind a reverse proxy that validates Host itself. Accepts 1 or true (case-insensitive); any other value is ignored. Takes precedence over ALLOWED_HOSTS.

When both unset (or ALLOWED_HOSTS is empty after trim), the rmcp loopback-only default applies unchanged.

Implementation: new module-level helper build_streamable_http_config() in src/serve/mod.rs encapsulates the resolution order (disable > custom > default). Called once from run_serve in place of the previous inline StreamableHttpServerConfig::default(). New constants ALLOWED_HOSTS_ENV and DISABLE_HOST_VALIDATION_ENV in src/constants.rs follow the existing pattern (ALLOWED_ROOTS_ENV, SERVE_API_KEY_ENV).

Tests: 7 unit tests in mod allowed_hosts_tests cover all branches:

• default loopback-only (both env vars unset)
• custom ALLOWED_HOSTS replaces default
• DISABLE=1 clears allowlist
• DISABLE=TRUE case-insensitive
• DISABLE=yes (other value) ignored
• empty ALLOWED_HOSTS falls back to default
• DISABLE takes precedence over ALLOWED_HOSTS

Validation

• cargo fmt --check clean
• cargo clippy --all-targets -- -D warnings clean
• cargo test --lib --bins: 1188 passed, 36 ignored, 0 failed

Files

File	Changes
src/constants.rs	+2 env var constants with doc-comments
src/search/mod.rs	1-line floor_char_boundary fix + regression test
src/serve/mod.rs	new build_streamable_http_config() helper + call-site swap + 7 unit tests

Closes #148.
Closes #149.