Fix issue where HTML file parsing may lose content. (#11536)

### What problem does this PR solve?

##### Problem Description
When parsing HTML files, some page content may be lost.  
For example, text inside nested `<font>` tags within multiple `<div>`
elements (e.g.,
`<div><font>Text_1</font></div><div><font>Text_2</font></div>`) fails to
be preserved correctly.

###### Root Cause #1: Block ID propagation is interrupted
1. **Block ID generation**: When the parser encounters a `<div>`, it
generates a new `block_id` because `<div>` belongs to `BLOCK_TAGS`.
2. **Recursive processing**: This `block_id` is passed down recursively
to process the `<div>`’s child nodes.
3. **Interruption occurs**: When processing a child `<font>` tag, the
code enters the `else` branch of `read_text_recursively` (since `<font>`
is a Tag).
4. **Bug location**: The first line in this `else` branch explicitly
sets **`block_id = None`**.
- This discards the valid `block_id` inherited from the parent `<div>`.
- Since `<font>` is not in `BLOCK_TAGS`, it does not generate a new
`block_id`, so it passes `None` to its child text nodes.
5. **Consequence**: The extracted text nodes have an empty `block_id` in
their `metadata`. During the subsequent `merge_block_text` step, these
texts cannot be correctly associated with their original `<div>` block
due to the missing ID. As a result, all text from `<font>` tags gets
merged together, which then triggers a second issue during
concatenation.
6. **Solution:** Remove the forced reset of `block_id` to `None`. When
the current tag (e.g., `<font>`) is not a block-level element, it should
inherit the `block_id` passed down from its parent. This ensures
consistent ownership across the hierarchy: `div` → `font` → `text`.

###### Root Cause #2: Data loss during text concatenation
1. The line `current_content += (" " if current_content else "" +
content)` has a misplaced parenthesis. When `current_content` is
non-empty (`True`):
    - The ternary expression evaluates to `" "` (a single space).
    - The code executes `current_content += " "`.
- **Result**: Only a space is appended—**the new `content` string is
completely discarded**.

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)

deepdoc/parser/html_parser.py

@@ -138,7 +138,6 @@ class RAGFlowHtmlParser:
                                             "metadata": {"table_id": table_id, "index": table_list.index(t)}})
                 return table_info_list
             else:
-                block_id = None
                 if str.lower(element.name) in BLOCK_TAGS:
                     block_id = str(uuid.uuid1())
                 for child in element.children:
@@ -172,7 +171,7 @@ class RAGFlowHtmlParser:
                 if tag_name == "table":
                     table_info_list.append(item)
                 else:
-                    current_content += (" " if current_content else "" + content)
+                    current_content += (" " if current_content else "") + content
         if current_content:
             block_content.append(current_content)
         return block_content, table_info_list