[type: bug] Fix thread safety issue with shared static CONTEXT in ExpressionGenerator (#6880)

[zhang-arvin]

Closes #6880

[Aias00]

Summary

Correct concurrency fix for the mock expression generators.

Root cause (confirmed)

ExpressionGenerator (and StandardExpressionGenerator) held a private static final EvaluationContext CONTEXT that was mutated per request via CONTEXT.setVariable("req", mockRequest) before getValue(CONTEXT). Because the context was shared and mutable, concurrent mock evaluations raced on the req variable (and any other context state), producing cross-request interference or inconsistent results.

What the fix does right

• Replacing the shared static with a fresh initContext() per doGenerate call eliminates the shared mutable state — each invocation gets its own EvaluationContext.
• The static SpelExpressionParser PARSER remains shared, which is safe: SpelExpressionParser and the parsed Expression objects are documented thread-safe, so the parser was never the problem — only the context.
• Verified initContext() returns a new EvaluationContext each call, and no other references to the removed static CONTEXT remain in either ExpressionGenerator or StandardExpressionGenerator.

Verdict

Approve. No blocking issues.