gopls/internal/golang: fall back when ( is not a dynamic function call

[manimz]

When implementation is requested at the "(" paren of a call, implFuncs
first checks whether the call is dynamic. For calls that are not dynamic
function calls (for example, method calls), that check fails.

Instead of returning a hard error, return errNotHandled so the request
falls back to the regular method-set implementation path.

This restores the fallback behavior that regressed in
25a90be.

Fixes golang/go#77784

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Patch Set 1:

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