…9581)

Refactor the generic `sy_services_linetype` rule into service-specific
rules to enable service-specific configuration parsing. This change:

1. Replaced generic `sy_services_linetype` + `line_type` with:
- `syserv_ftp` for FTP service
- `syserv_ssh` for SSH service
- `syserv_telnet` for TELNET service

2. Renamed `sy_services_null` to `syserv_null` for naming consistency

3. Created SSH-specific `syservs_null` rule (currently identical to
`sysl_null` but will diverge when SSH-specific options are added)

4. Kept `sysl_null` for legacy use by FTP/TELNET with TODO comments
for future service-specific refactoring

This separation enables service-specific parsing while maintaining
backward compatibility. Reference tests updated to reflect new
parse tree structure.

---
Prompt:
```
so, I don't like that we have a generic sysl where we mix different line types. Let's do a minimal refactoring to extract ssh. I think we get rid of system_line_type and replace it with 3 rules for the three line types. The non-SSH ones can stay sysl_null as their body with a comment to improve this later with line-type-specific code. Then let's move all the stuff we just added into system_ssh-specific rules and not in sysl_null
```

Add parsing support for SSH boolean configuration options:
- access-disable-external
- allow-tcp-forwarding (replacement for deprecated tcp-forwarding)
- no-challenge-response
- no-password-authentication
- no-passwords (already existed in sysl_null)
- no-public-keys
- no-tcp-forwarding (deprecated, kept for compatibility)
- root-login (with null_filler to consume the login mode parameter)
- tcp-forwarding (deprecated, kept for compatibility)

Each option is parsed as a distinct grammar rule with _null suffix
rather than being grouped in a generic _null rule. This provides
grammatical precision while still indicating the options are not
extracted into the Batfish data model.

The `syservs_null` rule is simplified to contain only the remaining
SSH options that don't yet have individual rules.

---
Prompt:
```
In exactly the same style as last commit, add support for `set system services ssh allow-tcp-forwarding`: https://www.juniper.net/documentation/us/en/software/junos/cli-reference/topics/ref/statement/ssh-edit-system.html . This is a large grammar, so implement only the things that are easy and obvious. Maybe just the booleans with no parameters, like no-passwords?
```