fix(config): return pointers to slice elements in LocalConfig getters

[Rahulatram321]

Description

This PR fixes pointer semantics in LocalConfig getters to ensure returned pointers reference real backing slice elements.

Problem

The getters used range-value iteration patterns like:

• for _, u := range l.Users { return &u }

This returns the address of a loop variable copy, not the original slice element.
As a result, callers mutating through returned pointers may silently modify detached copies, causing subtle state bugs.

Changes

Updated all affected getters in pkg/config/localconfig.go to index-based iteration:

• GetUser
• GetServer
• GetInstance
• GetAuth

Pattern changed to:

• for i := range l.Users { return &l.Users[i] } (and equivalent for other slices)

Tests

Added pkg/config/localconfig_test.go with regression coverage that verifies pointer-backed mutation for all four getters:

• mutate field through returned pointer
• assert corresponding slice element is updated

Result

• Getter pointers now correctly point into LocalConfig slices.
• Prevents stale-copy mutation bugs and makes behavior consistent with caller expectations.