fix(session): never send an empty-content user message (Bedrock 400)

[wqymi]

Summary

• Root cause: toModelMessagesEffect in message-v2.ts:681 pushed a UIMessage with parts: [] into the result array before the loop populated parts. If all DB parts were filtered (ignored text, unsupported file types, etc.), the empty user message leaked through to convertToModelMessages, producing a ModelMessage with empty content that Bedrock/Anthropic reject with 400.
• Secondary gap: normalizeMessages in transform.ts:64 passed through non-string/non-array content (null, undefined) without checking, allowing empty-content user messages to bypass the Anthropic/Bedrock filter.

Changes

packages/opencode/src/session/message-v2.ts

Build user message parts into a local userParts array, then only push the UIMessage when userParts.length > 0. Eliminates the push-before-populate race where an empty parts: [] message could leak through.

packages/opencode/src/provider/transform.ts

Added explicit guard at the top of normalizeMessages: user messages with null, undefined, or empty-array content are dropped before reaching the provider. Safety-net backstop for any future code path that might produce empty-content user messages.

packages/opencode/test/provider/transform.test.ts

3 new regression tests:

• Filters user messages with null content for Bedrock
• Filters user messages with empty array content for Bedrock
• No user message in the built request has empty content (Bedrock regression — comprehensive sweep of empty string, null, empty array, empty parts)

Verification

• bun typecheck passes (turbo typecheck: 12/12 packages)
• All 212 transform tests pass (including 3 new)
• All 26 message-v2 tests pass
• All 5 message-v2-filter tests pass