sway-core: Clarify error for self used without a self parameter

[Dnreikronos]

Closes #7645

Using self as a value inside a method that has no self parameter reported "Identifier "self" was used as a variable, but it is actually a generic type parameter." That is misleading: self is a keyword, not a user generic. It happens because both self and Self get inserted into the namespace as the injected Self type parameter, so resolution falls through to it once no real self parameter shadows the name.

Special-case that resolution and emit a new SelfParameterNotAvailable error pointing at the missing self parameter. Self and methods that do take self are unaffected, since a real self parameter resolves to a variable. Adds a should_fail test covering both the method-receiver and bare-value uses.

[fuel-cla-bot]

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[cursor]

PR Summary

Low Risk
Compiler-only diagnostic change on a narrow name-resolution path; behavior for valid self usage is unchanged.

Overview
Improves compile diagnostics when self is used as a value in an impl method that does not declare a self parameter. Without a real self binding, name resolution used to hit the injected Self generic (GenericTypeForFunctionScope) and report NotAVariable (“actually a generic type parameter”), which was confusing.

Variable expression type-checking now special-cases self resolving to that generic and emits SelfParameterNotAvailable with a message that the enclosing function lacks a self parameter. Self as a type and methods that do take self are unchanged. A should_fail e2e test covers both self.x() and bare self uses.

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[Dnreikronos]

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✅ Bugbot reviewed your changes and found no new issues!

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[codspeed-hq]

Merging this PR will not alter performance

✅ 25 untouched benchmarks

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_{Comparing Dnreikronos:typecheck/self_param_missing_error (44836c2) with master (ac933ff)}