fix(compiler): enter template scope before resolving operation param defaults

[timotheeguerin]

Fixes #11454

Problem

checkOperation resolves template parameter defaults (via checkTemplateDeclaration) before entering the InTemplateDeclaration scope — unlike checkModelStatement and interface/union checks, which set the flag first.

As a result, when a decorated template is used as an operation template parameter default (the ARM tags-update pattern, e.g. op foo<Resource, Properties extends {} = TagsUpdateModel<Resource>>(...)), the default is instantiated with Resource still an unresolved TemplateParameter, and skipDecorators is false. The decorators on the default type (@friendlyName, @doc, …) therefore run with an unresolved template parameter argument and store unresolved {name} strings. In @typespec/http-server-csharp this surfaced as a model file named {name}TagsUpdate.cs instead of FooResourceTagsUpdate.cs.

A decorator should never run with a TemplateParameter argument — the real bug is the ordering, not the emitter.

Fix

In checkOperation, set CheckFlags.InTemplateDeclaration before calling checkTemplateDeclaration, mirroring checkModelStatement. Operations now enter the template declaration scope before resolving parameter defaults, so decorators on those defaults are no longer executed with the still-unresolved template parameter.

Tests

• compiler: regression test using a custom @track decorator that records the kind of its argument. It asserts @track never runs with a TemplateParameter (only the concrete Model). Verified: fails before the fix, passes after.
• http-server-csharp: integration test using the real operation default-arg pattern, asserting no emitted file name contains an unresolved {/} placeholder.

Background

This supersedes and replaces #11459, which had diverged from its original purpose and accumulated several earlier band-aid attempts. This PR contains only the root-cause compiler fix plus tests.

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commit: 56074aa

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All changed packages have been documented.

• ✅ @typespec/compiler

Show changes

@typespec/compiler - fix ✏️

Fix decorators running with unresolved template parameters when a decorated template is used as a template parameter default of an operation (e.g. op foo<Resource, Properties = Decorated<Resource>>(...), the ARM TagsUpdateModel<Resource> pattern). Operations now enter the template declaration scope before resolving template parameter defaults, so decorators on those defaults are no longer executed with the still-unresolved template parameter. This matches the existing behavior for models and interfaces.

@typespec/http-server-csharp - fix ✏️

Fix model file name for @friendlyName ARM-style template patterns (e.g. @friendlyName("{name}TagsUpdate", Resource) on TagsUpdate<Resource>). The instantiation now correctly receives the substituted name (e.g. FooResourceTagsUpdate.cs) from the compiler.

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