fix(describe-affected): use correct context data when rendering spacelift name_template admin-stack prefix

[coderabbitai]

Summary

In name_template mode, addAffectedSpaceliftAdminStack in internal/exec/describe_affected_utils_2.go renders both the admin-stack selector prefix (line 473) and the per-candidate stack prefix (line 512) against the same configAndStacksInfo.ComponentSection. Because both sides of the comparison always produce the same string, the adminStackContextPrefix == contextPrefix guard at line 523 is effectively a no-op, and the function can match the wrong admin stack when a component name repeats across stacks.

This is in contrast to the name_pattern else branches, which correctly use adminStackContext (the selector) for the admin-stack prefix and the candidate's own context for the per-candidate prefix.

Steps to reproduce

1. Configure stacks.name_template (instead of stacks.name_pattern).
2. Have two stacks that share a component name, where one stack has settings.spacelift.admin_stack_selector pointing to a specific admin component.
3. Run atmos describe-affected --include-dependents.
4. Observe that the admin stack may be incorrectly identified because the prefix comparison is always equal.

Expected behavior

• The admin-stack prefix should be rendered using the selector context (adminStackContext / adminStackContextSection).
• The per-candidate prefix should be rendered using the candidate component's own section (the iterated componentSection).

Proposed fix

- adminStackContextPrefix, err = ProcessTmpl(atmosConfig, "spacelift-admin-stack-name-template", atmosConfig.Stacks.NameTemplate, configAndStacksInfo.ComponentSection, ...)
+ adminStackTemplateData := map[string]any{
+     cfg.VarsSectionName: adminStackContextSection,
+ }
+ adminStackContextPrefix, err = ProcessTmpl(atmosConfig, "spacelift-admin-stack-name-template", atmosConfig.Stacks.NameTemplate, adminStackTemplateData, ...)
...
- contextPrefix, err = ProcessTmpl(atmosConfig, "spacelift-stack-name-template", atmosConfig.Stacks.NameTemplate, configAndStacksInfo.ComponentSection, ...)
+ contextPrefix, err = ProcessTmpl(atmosConfig, "spacelift-stack-name-template", atmosConfig.Stacks.NameTemplate, componentSection, ...)

A fixture test covering name_template + spacelift admin_stack_selector + describe-affected-with-dependents should be added.

Context

• Identified during review of PR fix(templates): honor ignore_missing_template_values for stack name_template (#2345) #2619 (comment: fix(templates): honor ignore_missing_template_values for stack name_template (#2345) #2619 (comment)).
• The data-source argument was pre-existing and out of scope for PR fix(templates): honor ignore_missing_template_values for stack name_template (#2345) #2619, which only routed IgnoreMissingTemplateValues to name-template call sites.
• Requested by Andriy Knysh (@aknysh).