fix: enforce const-correctness in indexed_vector

[vietcgi]

Problem

Const at() methods were returning mutable pointers via const_cast, violating const-correctness.

Solution

• Split at() methods into const/non-const overloads
• Const version returns const T*, non-const returns T*
• Updated callers to use const pointers where appropriate
• Removed unsafe const_cast

Changes

• indexed_vector.h: Added const/non-const overloads for both at() methods
• rule_loader_compiler.cpp: Updated helper functions to return const pointers

Verified compilation and all call sites handle const-correctness properly.

[poiana]

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[poiana]

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This pull-request has been approved by: vietcgi
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[github-actions]

This PR may bring feature or behavior changes in the Falco engine and may require the engine version to be bumped.

Please double check userspace/engine/falco_engine_version.h file. See versioning for FALCO_ENGINE_VERSION.

/hold