fix(cli): allow PDF uploads

[imetandy]

Summary

• allow application/pdf through the CLI upload MIME gate
• render PDFs as file attachments rather than broken inline images
• preserve the original filename in imeta metadata
• use the relay-aligned 100 MiB generic-file size limit for PDFs
• add regressions for PDF upload, rendering, filename metadata, image rendering, and size selection

The relay already accepts PDFs on its generic file path; the CLI was rejecting them before sending the request.

Fixes #2963.

Closest existing work: #6364 is broader and currently blocked; this PR is the minimal PDF-only fix for a reproduced user-facing failure.

Verification

• cargo test -p buzz-cli — 467 passed
• cargo clippy -p buzz-cli --all-targets -- -D warnings — passed
• cargo fmt -p buzz-cli — passed
• git diff --check — passed

[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-08T07:27:46.931979Z	477035a	PR opened

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review" or "@codex security review".

Codex reacts with 👀 while any review is running, comments if it has suggestions, and reacts with 👍 once all reviews finish with no findings.

[github-actions]

🔐 Codex Security Review

Status: review required for the current range.

The current range is 3c7f288c60d67df78577b237e27c3dfc8831aaa1...aa70af75fc1f8689a2504c17012dca747c986352.
A new review must complete for this exact range. When manual authorization
is required, a Block organization member must comment exactly
@buzz-security-review aa70af75fc1f8689a2504c17012dca747c986352 to authorize a new review.
Any previous review applies only to its recorded range.

[chatgpt-codex-connector]

💡 Codex Review

Here are some automated review suggestions for this pull request.

Reviewed commit: 477035af57

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".