fix(sessions): keep inlineData mimeType when encoding or decoding content

[hermanrous]

What

SessionUtils.encodeContent and SessionUtils.decodeContent rebuilt every inline blob from scratch:

part.toBuilder().inlineData(Blob.builder().data(encodedData).build()).build()

Blob.builder().data(...) produces a Blob carrying only data, so mimeType and displayName are silently discarded. com.google.genai.types.Blob documents mimeType as "Required. The IANA standard MIME type of the source data.", and this is the live persistence path for Vertex AI sessions:

VertexAiSessionService.appendEvent → SessionJsonConverter.convertEventToJson → SessionUtils.encodeContent

and back on reload via fromApiEvent → SessionUtils.decodeContent.

So every multimodal event (image / audio / PDF inlineData) stored to Vertex AI loses its MIME type permanently, and decodeContent(encodeContent(content)) does not return the input either.

adk-python preserves inline_data.mime_type on the same path (model_dump(mode='json'), and _drop_vertex_unsupported_part_fields only pops part_metadata), and CONTRIBUTING.md names adk-python as the source of truth for validation.

How

Reuse the existing blob's builder instead of constructing a fresh Blob:

part.toBuilder().inlineData(inlineDataBlob.toBuilder().data(encodedData).build()).build()

The fields that are not being rewritten (mimeType, displayName) are preserved; displayName comes back for free, which is the same class of loss.

Backward compatible: the stored JSON simply gains a mimeType field that adk-python already writes, and sessions written before this change still read fine (mimeType stays absent, exactly as today).

Tests

New SessionUtilsTest covers encode, decode, the round trip, and the JSON that is actually persisted:

test	before	after
encodeContent_inlineData_preservesMimeType	FAIL	pass
decodeContent_inlineData_preservesMimeType	FAIL	pass
decodeContent_encodeContent_roundTripsInlineData	FAIL	pass
convertEventToJson_inlineData_persistsMimeType	FAIL	pass

On unmodified main all four fail:

[ERROR] Tests run: 4, Failures: 4, Errors: 0, Skipped: 0
[INFO] BUILD FAILURE

With the change applied:

[INFO] Tests run: 4, Failures: 0, Errors: 0, Skipped: 0
[info] Processed 360 files (0 reformatted).
[INFO] BUILD SUCCESS

Verified with mvn -B -pl core -am -Dtest=SessionUtilsTest test on Java 21.

Note

I used an AI assistant to draft this change. I reviewed it, ran the build and the tests locally, and verified each claim in this description myself.

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