fix(agentengine): base64-encode []byte fields instead of failing thei…

[rizukijr]

Link to Issue or Description of Change

1. Link to an existing issue (if applicable):

• Closes: []byte fields (e.g. genai.Part.ThoughtSignature) fail helper.ConvertSnake with "unsupported type: uint8", breaking Agent Engine event JSON #1386

Testing Plan

Unit Tests:

• I have added or updated unit tests for my change.
• All unit tests pass locally.

Added two tests to encode_test.go, following the existing file's conventions:

• TestByteSlice - a session.Event with a non-empty
  Content.Parts[0].ThoughtSignature now converts successfully, and the
  resulting thought_signature key holds the expected base64 string
  ({0xDE, 0xAD, 0xBE, 0xEF} -> "3q2+7w==", matching what
  encoding/json produces for the same bytes).
• TestByteSliceOmitEmpty - confirms omitempty is unaffected: an empty
  []byte is still correctly omitted, not encoded as an empty string.

Ran the full existing package suite alongside the new tests - zero regressions:

$ go test ./server/agentengine/...
ok      google.golang.org/adk/v2/server/agentengine/controllers/method  0.972s
ok      google.golang.org/adk/v2/server/agentengine/internal/helper     0.491s
$ go vet ./server/agentengine/...
(clean)

Manual End-to-End (E2E) Tests:

Ran the real agentengine launcher (web agentengine - the same launcher
composition agentengine.NewLauncher uses in production) locally against a
real deployed, Vertex AI Sessions-backed Reasoning Engine resource, using
gemini-3.6-flash (emits a non-empty ThoughtSignature on essentially
every turn, not just tool calls - the worst case observed). Sent the exact
async_stream_query request shape Gemini Enterprise uses:

curl -s -X POST -H "Content-Type: application/json" \
  "http://localhost:8080/api/stream_reasoning_engine" \
  -d '{"class_method": "async_stream_query", "input": {"user_id": "verify", "message": {"role": "user", "parts": [{"text": "hi"}]}}}'

• Before the fix: every signature-bearing event logged Failed to convert: ... err: unsupported type: uint8 and shipped to the client in raw
  Go/camelCase field names (thoughtSignature, finishReason,
  invocationId) mixed into an otherwise snake_case stream.
• After the fix: zero Failed to convert log lines. The event now
  serializes correctly, e.g. "thought_signature": "AY89a1...", with
  consistent snake_case throughout (finish_reason, invocation_id, etc).

Also confirmed this is not model- or version-specific: the same missing
reflect.Uint8 case is present identically in all three published
adk/v2 releases (v2.0.0, v2.1.0, v2.2.0), and the bug reproduces on
gemini-2.5-flash too on any function-calling turn (just not on every
turn, unlike gemini-3.6-flash). Full details, including the live Gemini
Enterprise retry-into-dead-session cascade this caused in production, are
in #1386.

Checklist

• I have read the CONTRIBUTING.md document.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• I have manually tested my changes end-to-end.

Additional context

convertSnake's type switch only ever handled signed integers
(Int/Int8/Int16/Int32/Int64) - there was no case for Uint8 or
any other Uint* kind. A []byte field is a reflect.Slice, so it fell
into the generic per-element loop, and each individual byte (a bare
uint8) hit the default: unsupported type branch, failing conversion of
the entire containing struct. encoding/json.Marshal already solves
this correctly (base64-encodes []byte automatically) - this fix just
applies the same, standard approach instead of reinventing a worse version
of it for one specific hand-rolled converter.