fix(memory): skip thought parts in in-memory search and preloaded memories - #1975

Linked issue

• Closes: memory: in-memory search and preloadmemorytool include the model's thought parts #1974
• Related: fix(preloadmemorytool): build memory queries from all text parts #1852

2. Describe the change:

Problem:
The in-memory memory service indexed thought parts for search and returned them in entry content, and preloadmemorytool wrote them into the <PAST_CONVERSATIONS> instruction. So the model's old reasoning was matched on and fed back as conversation.

Solution:
Skip Thought parts in AddSessionToMemory (for words and for the stored content, which is now a copy with only the visible parts) and in preloadmemorytool's extractText. Same as adk-python b138b5a6a.

Behavior change

Search no longer matches words that only appear in thought parts, returned entries don't include thought parts, and the preloaded instruction leaves them out. An event with only thought text is no longer stored in memory.

Testing Plan

Unit Tests:

• All unit tests pass locally.

Added Test_inMemoryService_SearchMemory_SkipsThoughtParts and two cases to TestPreloadMemoryTool_ProcessRequest (mixed entry, thought-only entry).

With your source change reverted and your tests kept, which test fails?

--- FAIL: Test_inMemoryService_SearchMemory_SkipsThoughtParts
    SearchMemory("secret plan") matched thought text, got 2 entries
--- FAIL: TestPreloadMemoryTool_ProcessRequest/thought_parts_are_omitted
--- FAIL: TestPreloadMemoryTool_ProcessRequest/thought-only_memory_entry

With the change, go test -race ./memory/... ./tool/... passes and golangci-lint is clean.

Manual End-to-End (E2E) Tests:

N/A, covered by the unit tests.

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.
• Any dependent changes have been merged and published in downstream modules.