fix(server): reject malformed UTF-8 webhook payloads

[xuedingjie]

Problem

readWebhookPayload decoded request bytes with the default TextDecoder, which replaces malformed UTF-8 sequences with U+FFFD. A malformed JSON request could therefore be accepted after its payload was silently rewritten, and a Run would be created with data different from the bytes sent by the caller.

Fix

Decode Webhook bodies with a fatal UTF-8 decoder and map decoding failures to the existing 400 invalid-request response. Add a regression test that sends an invalid UTF-8 sequence and verifies that no Run is created.

This matches the strict decoder already used by the Integration callback path and the JSON UTF-8 requirement in RFC 8259 section 8.1.

Verification

• npx --yes bun@1.4.0 x vitest run --config vitest.config.ts test/webhook.test.ts
• npx --yes bun@1.4.0 x oxfmt --check apps/server/node/http.ts apps/server/test/webhook.test.ts
• npx --yes bun@1.4.0 x oxlint --deny-warnings apps/server/node/http.ts apps/server/test/webhook.test.ts
• npx --yes bun@1.4.0 x tsc --noEmit -p apps/server/tsconfig.json

[coderabbitai]

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📥 Commits

Reviewing files that changed from the base of the PR and between 59bec4a and fc300a3.

📒 Files selected for processing (2)

• apps/server/node/http.ts
• apps/server/test/webhook.test.ts

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