fix: add a special case for item variable during global expression replacement

[elliotgunton]

Fixes #15008

Motivation

• The root cause of the issue was from SubstituteParams, where the globalReplacedTmplStr would have already lost the ={{sprig.func(item)}} expression which became an empty string. This meant the later processItem function had no expression to expand.
• This seems to have come from a change in sprig which in turn came from the expr library upgrade from 1.16 to 1.17 in 9b7c0c4

Modifications

• We now have a special case to not attempt to run the expr program as an unresolved item in a sprig string function now comes back as an empty string, instead of nil.
• As part of this change, refactored the anyVarNotInEnv to return the varname that is missing as a reference to a string, or nil.
• Also removed a completely unused function hasRetries and refactored and expanded the unit tests where it was used
• Added an e2e test using the workflow from the issue

Verification

• e2e test added using the workflow from the issue
• Also tested manually.

[Joibel]

nits only

[Copilot]

Pull Request Overview

This PR fixes an issue where the item variable in sprig string functions (like sprig.upper(item)) was being incorrectly resolved to an empty string during global expression replacement, causing withItems loops to fail. The fix adds special handling for the item variable alongside existing special cases for retries, lastRetry.*, and workflow.* variables.

• Refactored anyVarNotInEnv to check against a centralized list of variables and return the missing variable name
• Consolidated multiple conditional checks into a single unified check with a variablesToCheck list
• Removed unused hasRetries function and improved test coverage

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
util/template/expression_template.go	Refactored variable checking logic to use a centralized list and unified handling for unresolved variables including item
util/template/expression_template_test.go	Renamed and expanded test from Test_hasRetries to Test_hasVariableInExpression with additional test cases for item variable
test/e2e/expr_lang_test.go	Added e2e regression test for issue #15008 and fixed typo in existing test suite name

Comments suppressed due to low confidence (1)

test/e2e/expr_lang_test.go:110

• Corrected spelling from 'TestExprLangSSuite' to 'TestExprLangSuite' (removed duplicate 'S').

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