Auto-escape special XML characters for from_env/from_zk substitutions

### Company or project name

_No response_

### Describe the unexpected behaviour

When using `from_env` or `from_zk` in configuration files, environment variables containing XML special characters (`&`, `<`, `>`, etc.) cause parsing failures. ClickHouse builds a temporary XML snippet like `...env_value...`, which breaks unless values are manually escaped or wrapped in ``.

### How to reproduce

Set an environment variable with XML-reserved characters, e.g. export `MY_ENV_VAR="&<>!"`.

In your config, use ``.

Start ClickHouse.

Observe the parsing error (Invalid token in line 1 column X).

### Expected behavior

ClickHouse should automatically escape special characters, ensuring valid XML is passed to the parser without requiring manual entity replacement or CDATA.

### Error message and/or stacktrace

```
Processing configuration file '/etc/clickhouse-server/config.xml'.
Merging configuration file '/etc/clickhouse-server/config.d/test.xml'.
Poco::Exception. Code: 1000, e.code() = 0, Exception: Failed to preprocess config '/etc/clickhouse-server/config.xml': SAXParseException: Invalid token in line 1 column 12, Stack trace (when copying this message, always include the lines below):

0. DB::ConfigProcessor::processConfig(bool*, zkutil::ZooKeeperNodeCache*, std::shared_ptr const&, bool) @ 0x0000000014664288
1. DB::ConfigProcessor::loadConfig(bool, bool) @ 0x0000000014664af7
2. BaseDaemon::initialize(Poco::Util::Application&) @ 0x000000000e07c0b9
3. DB::Server::initialize(Poco::Util::Application&) @ 0x000000000dfcce1b
4. Poco::Util::Application::run() @ 0x000000001748045a
5. DB::Server::run() @ 0x000000000dfccb30
6. mainEntryClickHouseServer(int, char**) @ 0x000000000dfc9ab3
7. main @ 0x0000000008baa8e1
8. ? @ 0x00007f1ddca111ca
9. ? @ 0x00007f1ddca1128b
10. _start @ 0x000000000677702e
(version 25.1.6.34 (official build))
```

### Additional context

W/A: Manually replace special characters with valid XML entities or use CDATA in environment variables.