Hrw: Supports nested If

[zwoop]

This refactors the parsing / evaluation, to put most of the exec / evaluation logic into
a new OperatorIf. This is not a real operator, that configurations would use, but allows
for the conditionals to be nested. The syntax in HRW is not awesome, but best we can
do within the constraints of backwards compatibility.

As such, this PR also includes changes and additions to hrw4u, where nested if's are
now supported and added to the existing if() / else semantics. E.g.

SEND_RESPONSE {
    if inbound.status == 200 {
        inbound.resp.X-When-200-Before = "Yes";
        if inbound.req.X-Foo == "foo" {
            inbound.resp.X-Foo = "Yes";
            if inbound.req.X-Bar == "bar" with NOCASE {
                inbound.resp.X-Foo-And-Bar = "Yes";
            } elif inbound.req.X-Fie == "fie" with NOCASE {
                inbound.resp.X-Foo-And-Fie = "Yes";
            }
        } elif inbound.req.X-Foo == "maybe" {
.
.
.

[Copilot]

Pull Request Overview

This PR adds support for nested if/elif/else conditionals in the header rewrite system by refactoring the conditional logic into a new OperatorIf pseudo-operator. The changes enable arbitrary nesting depth for complex conditional logic while maintaining backward compatibility.

Key changes:

• Introduced OperatorIf class to encapsulate conditional evaluation logic
• Added if/endif keywords to enable nested conditional blocks
• Implemented explicit slot assignment syntax for variables (e.g., @7) in hrw4u
• Updated grammar files and parsers to support new syntax

Reviewed Changes

Copilot reviewed 43 out of 44 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
tools/hrw4u/grammar/hrw4u.g4	Added AT token and optional slot syntax to variable declarations
tools/hrw4u/grammar/u4wrh.g4	Added IF_OP and ENDIF_OP tokens and corresponding grammar rules
tools/hrw4u/src/visitor.py	Updated to handle explicit slot assignments and nested conditionals
tools/hrw4u/src/symbols.py	Refactored slot allocation to support explicit slot assignment
tools/hrw4u/src/types.py	Renamed index to slot for clarity
tools/hrw4u/src/hrw_visitor.py	Added support for if/endif operators and fixed nesting depth tracking
tools/hrw4u/scripts/testcase.py	Added exceptions mechanism to skip direction-specific tests
plugins/header_rewrite/ruleset.h	Refactored to delegate conditional logic to OperatorIf
plugins/header_rewrite/ruleset.cc	Simplified execution to use OperatorIf::exec_and_return_mods
plugins/header_rewrite/operators.h	Added OperatorIf class and OperatorAndMods struct
plugins/header_rewrite/operators.cc	Implemented OperatorIf conditional evaluation logic
plugins/header_rewrite/operator.h	Moved OperatorAndMods to operator.h for shared use
plugins/header_rewrite/parser.h	Added IF and ENDIF clause types
plugins/header_rewrite/parser.cc	Added parsing support for if/endif keywords
plugins/header_rewrite/header_rewrite.cc	Implemented if/endif stack-based parsing with nesting validation
doc/admin-guide/plugins/header_rewrite.en.rst	Added comprehensive documentation for nested conditionals
doc/admin-guide/configuration/hrw4u.en.rst	Documented explicit slot assignment syntax
tests/gold_tests/pluginTest/header_rewrite/*	Added test cases for nested conditionals
tools/hrw4u/tests/data/*	Added test cases for explicit slots and updated error messages

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}

[zwoop]

Ignored the nitpick comments, the current code is inline with other uses within the code base. Consistency is better.