security(api): reject non-text controls in multiline SQL requests

[seonghobae]

Outcome

Reject non-text C0/C1 control characters in multiline ApplySqlIn.sql requests before the API handler or DDL validator runs. Horizontal tab, LF, and CR remain valid multiline text whitespace.

Closes #764.

Security boundary

• Applies at the Pydantic request-model boundary.
• Rejects C0 controls other than \\t, \\n, and \\r.
• Rejects DEL and the C1 range (U+007F..U+009F).
• Does not expand the accepted SQL grammar or execution authority.

Evidence

• RED: eight representative C0/C1 inputs were accepted before the validator (8 expected failures); CR/LF/tab passed.
• GREEN: 22 passed in tests/test_api_apply_sql.py.
• Full backend: 388 passed, 1 skipped (proxy variables removed because the sandbox injects a SOCKS proxy while the hash-locked CI environment does not).
• Mypy: clean across 68 source files.
• Docstring contract: 1 passed.
• Exact owned coverage: app/schemas.py 150/150 statements and 4/4 branches (100%).
• git diff --check: clean.

Documentation

CHANGELOG.md and docs/api-security-checklist.md record the implemented request boundary and point to its code/tests.

Local tooling limitation

Repository guidance requests CodeGraph initialization/sync, but the executable is unavailable in this environment; direct rg symbol and caller search was used instead.

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📥 Commits

Reviewing files that changed from the base of the PR and between 72afe6d and c00edbc.

📒 Files selected for processing (4)

• CHANGELOG.md
• backend/app/schemas.py
• backend/tests/test_api_apply_sql.py
• docs/api-security-checklist.md

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[seonghobae]

Independent exact-head review requested for c00edbcffa28f59f32cc6aeab8375b062d56a04c.

@opencode-agent @cwl-noema-review Please review the request-model security boundary, the C0/C1 allow/deny classification, tests, and documentation. Do not mutate the branch or merge. The current CI, Security Scan, and SAST Semgrep runs are queued and are not being treated as passing; CodeRabbit's automatic comment is rate-limited and is not a review.

[seonghobae]

Superseded by canonical PR #832. The unique C1-range coverage from this duplicate has been ported test-first to #832 exact head 5bf6cc56ca4e80d89fed852362a6ae4f8717be98, which also retains the broader raw-validation-response redaction and transport documentation. Closing this PR to preserve a single writer lane for issue #764; no merge or branch-protection action is performed.