Return errors for missing infe content types

[kixelated]

Split from #199. Refs #179.

What changed

• Return Error::MissingContent when encoding a version 0 infe entry without its required content type.
• Return the same error for mime entries without a content type.
• Avoid cloning optional strings while encoding.
• Add regression tests for both invalid encoder states.

Why

These required fields were unwrapped during encoding, so invalid caller-provided state could panic instead of returning a normal error.

Impact

Encoding malformed infe entries now fails predictably without changing valid output.

Validation

• cargo fmt --all -- --check
• cargo test --all-features (252 unit tests and 3 doctests passed)
• cargo clippy --all-targets --all-features -- -D warnings
• git diff --check main...HEAD

[bradh]

Suggested change

	"content_type required for infe version 0",
	"content_type required for infe version 0 or 1",