Closes#6541
### What problem does this PR solve?
Add content validation to `update_chunk` (SDK and non-SDK) to reject
empty or whitespace-only content before it reaches the embedding model.
**Before:** Calling `update_chunk` with space-only content (like `" "`,
`""`, `"\n"`) bypassed validation and was sent directly to the embedding
model, which returned an error. This was the same bug previously fixed
for `add_chunk` in #6390, but `update_chunk` was missed.
**After:** Empty/whitespace-only content is caught by validation and
returns an error: `` `content` is required ``
### Type of change
- [x] Bug Fix (non-breaking change which fixes an issue)
### What problem does this PR solve?
- rename rmSpace to remove_redundant_spaces
- move clean_markdown_block to common module
- add unit tests for remove_redundant_spaces and clean_markdown_block
### Type of change
- [x] Refactoring
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Signed-off-by: Jin Hai <haijin.chn@gmail.com>
