Problem

The PromQL parser allowed invalid syntax like:

metric offset 5m[2m:10s]  # Should error, but was accepted
metric @ 123[2m:10s]      # Should error, but was accepted

While correctly rejecting the same pattern for regular range selectors:

metric offset 5m[2m]  # Correctly rejected

Solution

Added validation in the subquery_expr grammar rule to reject offset/@ modifiers when they appear immediately adjacent to the subquery bracket (no space between them).

The fix uses position checking (vs.PosRange.End == $2.Pos) to distinguish between:

• metric offset 5m[2m:10s] → Error (adjacent tokens, invalid)
• metric offset 5m [2m:10s] → Valid (spaced, subquery on offset result)

This makes subquery parsing behavior consistent with matrix_selector rules.

Testing

• Added test case for some_metric OFFSET 1m[5m:10s] expecting parse error
• Verified runtime behavior with API calls
• All offset-related tests pass
• Preserves valid use cases with spacing

Which issue(s) does the PR fix:

Fixes #17060

Does this PR introduce a user-facing change?

[BUGFIX] PromQL: Reject offset and @ modifiers immediately before subquery range selectors (e.g., `metric offset 5m[2m:10s]`). Valid syntax with spacing still works (e.g., `metric offset 5m [2m:10s]`).