Handle route callbacks that reference this during code splitting #8688

Summary

Route callbacks that reference this have an existing code-splitting problem. This affects function expressions as well as method-shorthand properties. Address this problem separately from the method-shorthand support in PR #8459.

Requested by @nstepien.

Required changes

• Establish the supported this contract for route callbacks that can be code-split.
• Evaluate enforcing this: void in the route callback types, as proposed in the discussion. This is a possible solution, not a predetermined requirement.
• If type restrictions do not fully address the problem, define appropriate compiler handling or diagnostics for unsupported callbacks.
• Add regression coverage for function expressions and method-shorthand callbacks that reference this, along with callbacks that do not reference this.
• Document any resulting callback restrictions.

Affected areas

• Router code-splitting transforms in packages/router-plugin/src/core/code-splitter/compilers.ts.
• Route callback type declarations.
• Router-plugin code-splitter tests.

Rationale

Extracting a callback into a split module must not silently change its supported this behavior. The problem predates method-shorthand support, so it should not block PR #8459.

Acceptance criteria

• The supported this behavior for split route callbacks is explicit.
• Unsupported this usage is rejected or diagnosed, or supported usage retains its behavior after code splitting.
• Regression tests cover both function expressions and method-shorthand callbacks.
• Callbacks that do not reference this continue to split correctly.

References

• PR: fix(router-plugin) code-splitter: add support for method shorthands #8459
• Scope and proposed approach: fix(router-plugin) code-splitter: add support for method shorthands #8459 (comment)
• Request to create this issue: fix(router-plugin) code-splitter: add support for method shorthands #8459 (comment)