fix(agent): use unique block ids for text/thinking stream events

[liulangjietou]

AgentScope-Java Version

2.0.0-RC4 (main, latest)

Description

ReActAgent'''s streaming block events for text and thinking blocks were emitted
with hardcoded block ids ("text" / "thinking"), which violates the documented
contract that blockId uniquely identifies each content block. Tool-call block
events in the same code path already use a unique toolId, so this brings
text/thinking blocks in line with that existing precedent.

Fix: ModelCallBlockLifecycle now lazily allocates a unique block id (same
generation scheme as replyId: UUID.randomUUID().toString().replace("-", ""))
when a text/thinking block starts, and reuses that id across the block'''s
Start/Delta/End events. Two hardcoded call sites were fixed — the primary
emitBlockEvents path and a second duplicate path further down the file.

No public API changes; no existing test asserted on the literal "text"/"thinking"
ids, so this is behavior-compatible for all in-repo consumers.

Fixes #1899

Checklist

• Code has been formatted with mvn spotless:apply
• All tests are passing (mvn test)
• Javadoc comments are complete and follow project conventions
• Related documentation has been updated (e.g. links, examples, etc.)
• Code is ready for review

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