fix(logging): document CHAT_LOG_MAX_BODY_KB, capture messageCount for Responses API bodies

[hartmark]

Summary

Extracted from PR #9439 (agentic conversation tracking). This was originally cherry-picked from a larger "make chat-log truncation configurable" commit, but most of that scope (the CHAT_LOG_MAX_BODY_KB env var support itself, the estimateSizeFast() earlyExitAt parameterization) turned out to already be present on the current upstream/release/v3.8.50 tip — confirmed by running the relevant tests against pristine upstream before making any changes here. Only two genuine gaps remained, both fixed:

1. Missing doc-sync entry. CHAT_LOG_MAX_BODY_KB was already read by getChatLogMaxBodyBytes() in code, but undocumented in .env.example and docs/reference/ENVIRONMENT.md. tests/unit/check-env-doc-sync.test.ts flags any env var read in code but missing from both doc files — documented it (both files required, the same test enforces the pairing).

2. messageCount never computed for Responses API bodies. truncateForLog()'s summary only computed messageCount from obj.messages (the OpenAI-chat/Gemini field name) — a large /v1/responses request (which uses input[], not messages[]) got summarized with no count at all, leaving the dashboard's "Full Conversation" panel nothing to base its "N messages not shown" placeholder on for any Responses-API conversation, even though the same summarization logic applies to it.

⚠️ base-red inherited: #9985

Test plan

• TDD: tests/unit/chatcore-log-truncation.test.ts's new regression test ("captures a message count for Responses API bodies too") — confirmed failing against the pre-fix code, passing after.
• tests/unit/check-env-doc-sync.test.ts confirms CHAT_LOG_MAX_BODY_KB no longer appears in codeMissingEnv (the remaining drift in that test — ANTIGRAVITY_ALLOW_SIGNATURE_BYPASS, COMMANDCODE_API_URL, OMNIROUTE_STRICT_SYSTEM_PROVIDERS, TLS_FINGERPRINT_PROVIDERS — is pre-existing/unrelated, confirmed identical on a pristine upstream/release/v3.8.50 checkout).
• tests/unit/chatcore-log-truncation.test.ts — 19/19 passing.
• npx tsc --noEmit / npm run lint — clean.

[diegosouzapw]

Verified: fix #2 (messageCount for /v1/responses input[] bodies) is real and TDD-proven — confirmed the pre-fix test fails and the post-fix test passes against a pristine base checkout. However, fix #1's premise doesn't hold: CHAT_LOG_MAX_BODY_KB was already documented in both .env.example (line ~2631, though with a stale comment pointing at a file that no longer exists, src/lib/chatLogTruncation.ts) and docs/reference/ENVIRONMENT.md (line ~1449, correctly referencing src/lib/logEnv.ts). Running check-env-doc-sync.mjs on the base tip confirms this var was never in the missing list. This PR adds a second, duplicate entry to each file rather than fixing the existing stale one — please consolidate into a single entry per file (and fix the stale file reference in .env.example) before merge. Not a functional blocker, just a documentation-accuracy nit worth a quick follow-up commit.

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Note: the base-red that was failing this PR's CI (#9985) was drained today — base-reds PR #10213 just merged into release/v3.8.50. A rebase/sync onto the current tip should bring your checks green.

[diegosouzapw]

Merged — thanks @hartmark! messageCount now lands for Responses input[] bodies; we consolidated the duplicate CHAT_LOG_MAX_BODY_KB doc entries on your branch before merging (kept your richer copy next to the CHAT_LOG_* family).