fix(cli): decode fixed32 OTLP span flags

[Makisuo]

Summary

• align the vendored OTLP trace schema with upstream by defining Span.flags and Link.flags as fixed32
• add a Python SDK-compatible protobuf fixture with a non-zero field-16 flags value
• verify the decoded request continues through Maple's trace encoder

Root cause

The local receiver declared the trace flags fields as uint32, while the OTLP schema defines them as fixed32. Real SDKs therefore emitted a different wire type, and protobuf decoding could desynchronize or reject spans whose sampled flag was set.

Impact

Local mode accepts sampled spans emitted by OpenTelemetry SDKs instead of returning HTTP 400, and preserves their flags value during decoding.

Validation

• bun --filter @maple/cli test — 101 passed
• bun --filter @maple/cli typecheck
• bunx oxfmt --check apps/cli/src/server/otlp/proto.ts apps/cli/src/server/otlp/encode.test.ts
• bunx oxlint apps/cli/src/server/otlp/proto.ts apps/cli/src/server/otlp/encode.test.ts

Closes #221

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✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.