feat(visitors): message and concurrent payload visitors

[jmaeagle99]

What was changed

Adds an SDK-internal payload/message visitor for Temporal API proto messages (io.temporal.internal.payload.visitor in temporal-sdk).

• PayloadVisitors.visit(message, options) — invokes a callback on every Payload reachable in any API message; the callback returns replacements, so payloads can be transformed in place.
• MessageVisitors.visit(message, options) — invokes a callback on each payload-bearing message (no per-payload visiting), for per-message-type logic.
• Per-message context seam (MessageVisitor) that threads a scoped value down the tree — e.g. tag payloads with the enclosing command's info.
• Bounded concurrency: sequential by default; concurrency > 1 requires a caller-supplied Executor.
• Handles all payload shapes, google.protobuf.Any, and cyclic messages (Failure).
• Visitor exceptions propagate unchanged to the caller.

The traversal code is generated at build time from the proto descriptors (new payloadVisitorGenerator source set + Gradle task; output under build/generated, not committed).

Why?

Provides the shared traversal foundation the upcoming external storage feature needs (concurrently offload large payloads), and the planned payload-limits validator.

Checklist

1. Closes Enable concurrent payload visiting - Java #2881 Allow payload visitors to visit memo payloads in aggregate - Java #2884

2. How was this tested: Unit tests under io.temporal.internal.payload.visitor (PayloadVisitorTest, MessageVisitorTest) covering traversal/mutation across every field shape, Any, cyclic messages, scoped context, concurrency, and error propagation

3. Any docs updates needed? No, internal implementation.