Fixes #3554

Motivation and Context

A tool annotated -> bytes publishes output_schema {"result": {"type": "string", "format": "binary"}}, but real binary data crashed the conversion instead of reaching the client: pydantic decodes bytes as UTF-8 when serializing to JSON, so the first non-UTF-8 byte raised PydanticSerializationError inside _convert_to_content (and UnicodeDecodeError from model_dump(mode="json")), and the client only saw a generic Error executing tool <name> with the payload lost. UTF-8-decodable bytes came back as a raw string rather than an encoding, and a bytes field inside an output model (e.g. class Thumb(BaseModel): data: bytes) hit the same crash.

This encodes bytes leaves as base64 — the encoding every other bytes path in this package already uses (BlobResourceContents at server.py:475, and Image/Audio content) — in both channels: the structured dump and the unstructured content. A payload without bytes takes the plain JSON dump, byte for byte.

How Has This Been Tested?

• uv run --frozen pytest tests/server/mcpserver/test_func_metadata.py tests/server/mcpserver/test_integration.py: 74 passed, including 6 new tests (binary and UTF-8 -> bytes in both channels, a bytes field in an output model, list[bytes] / dict[str, bytes], and a no-bytes regression guard) plus an end-to-end in-memory Client(server) call of a -> bytes tool.
• Full suite (uv run --frozen pytest): 5966 passed, 2 failed. Both failures reproduce on unpatched main on Windows (verified with git stash): test_sse_client_closes_all_streams_on_connection_error and test_safe_join_rejects_symlink_escape (WinError 1314, symlink privilege) — unrelated to this change.
• ruff check / ruff format --check: clean. pyright (strict): no new errors (34 before, 34 after; all pre-existing Windows-platform diagnostics).

Breaking Changes

None for payloads without bytes. Tools returning bytes now deliver base64 in structuredContent and in their text content, where binary data previously failed the call and UTF-8 bytes previously arrived as a raw string.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist

• I am assigned to the linked issue (or it is labeled help wanted, or I'm a maintainer)
• I have disclosed any AI assistance and can explain the change in my own words

AI assistance disclosure: the diff was written with the ZCode coding agent; I verified the reproduction on Windows (both failure paths), ran the test suites and linters listed above, and reviewed the serializer behaviour against server.py:475 / Image/Audio before submitting.