[BUG] Shared KB: file-only modifications clear knowledge_base_id and skip ingestion

## 🐞 Describe the bug

**Shared KB bots lose their `knowledge_base_id` after file-only modifications (v3 with shared KB feature)**

When a shared KB bot is modified by adding/removing files without changing the KB configuration (embeddings model, chunking strategy, etc.), two bugs occur:

1. `modify_owned_bot` calls `model_copy(update=modify_input.bedrock_knowledge_base.model_dump())` which overwrites `knowledge_base_id` with `None` (frontend sends `knowledgeBaseId: null`)
2. `is_sync_shared_knowledge_bases_required` returns `False` (hash unchanged) → `SyncSharedKnowledgeBasesRequired=False` → `FinalizeSharedKnowledgeBasesBuild` never runs → `knowledge_base_id` is never restored

The bot permanently loses its KB association in DynamoDB after any file-only edit.

## 🔄 To Reproduce

1. Create a shared KB bot with a file — KB syncs successfully, `knowledge_base_id` is set
2. Edit the bot to add another file (no KB config changes)
3. Observe `knowledge_base_id = null` in DynamoDB after the edit
4. Bot can no longer query its knowledge base

## 🔎 Root Cause

In `backend/app/usecases/bot.py`:
```python
updated_kb = current_bot_kb.model_copy(
update=modify_input.bedrock_knowledge_base.model_dump() # overwrites knowledge_base_id with None
)
```

In `backend/app/routes/schemas/bot.py`, `is_sync_shared_knowledge_bases_required` only returns `True` when the KB config hash changes — file-only changes return `False`, skipping `FinalizeSharedKnowledgeBasesBuild` which is the only step that restores `knowledge_base_id`.

## ✅ Suggested Fix

1. In `modify_owned_bot`, preserve `knowledge_base_id` and `data_source_ids` when merging KB config:
```python
updated_kb = current_bot_kb.model_copy(
update={
k: v for k, v in modify_input.bedrock_knowledge_base.model_dump().items()
if k not in ("knowledge_base_id", "data_source_ids") or v is not None
}
)
```

2. In `is_sync_shared_knowledge_bases_required`, always return `True` for shared KB bots so `FinalizeSharedKnowledgeBasesBuild` always runs:
```python
if self.bedrock_knowledge_base.type == "shared" or current_bot_model.bedrock_knowledge_base.type == "shared":
return True
```

## Environment

- Version: v3 (shared KB feature)
- Affected: `backend/app/usecases/bot.py`, `backend/app/routes/schemas/bot.py`

Start in backend/app/usecases/bot.py at modify_owned_bot and then inspect is_sync_shared_knowledge_bases_required in backend/app/routes/schemas/bot.py. Trace the shared knowledge-base edit flow and its finalization step; done means file-only modifications preserve knowledge_base_id and still restore or retain the association so the bot can query its knowledge base.

Written by the indexing model from the issue text.