[WRONG BRANCH] fix(vision): bound validation error prompts

[luvs01]

Motivation

• A data-URL mime validation path could reflect an unbounded attacker-controlled media-type string into the replacement prompt, allowing prompt/context amplification and excessive provider cost or latency.
• The goal is to cap and sanitize any validation/error text and to enforce a small, well-formed MIME syntax/length limit so errors cannot become large attacker-controlled text in the main provider prompt.

Description

• Enforce a MIME syntax check and a MAX_MIME_CHARS (127) bound in validateImageUrl() and return a small canonical error ("invalid image media type") for oversized or syntactically invalid media types (file: src/vision/describe.ts).
• Add boundedError() in src/vision/index.ts to strip control characters and clamp error text to DESC_MAX_CHARS, and use it when rendering sidecar errors so they cannot expand the upstream prompt.
• Preserve the existing per-image clamping for successful descriptions and ensure error markers are treated the same way before insertion into message parts.
• Add a regression test in tests/vision-cache.test.ts that verifies oversized data-URL media types are rejected without echoing long input and that thrown/sidecar errors are sanitized and size-bounded.

Testing

• node_modules/@oven/bun-linux-x64-baseline/bin/bun test tests/vision-cache.test.ts — passed (focused regression tests succeeded).
• PATH="$PWD/node_modules/@oven/bun-linux-x64-baseline/bin:$PATH" bun run test — passed (full test suite run with the pinned baseline Bun binary succeeded).
• bun run typecheck — succeeded (typecheck passed).
• bun run privacy:scan — succeeded (privacy scan passed).
• Note: the system-installed Bun (bun test tests/vision-cache.test.ts) was an older runtime (1.2.14) which produced an unrelated node:zlib export mismatch, so the repository-pinned Bun binary was used for reliable test execution.

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📥 Commits

Reviewing files that changed from the base of the PR and between 121f1ad and 0b9f5d7.

📒 Files selected for processing (3)

• src/vision/describe.ts
• src/vision/index.ts
• tests/vision-cache.test.ts

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[github-actions]

✅ Deterministic PR hygiene checks passed.

[github-actions]

⏳ DRAFT

• wrong target branch (main); retarget to dev.

What to do

• Retarget this PR to dev — all contributions go to dev.

Its title has been prefixed with [WRONG BRANCH].
This pull request was already a draft. Its draft status will be preserved after every issue above is resolved.