Rename vestigial `*modular*` vitest project and folder names in typespec-ts

## Context

RLC support has been dropped and the TypeScript emitter (`@azure-tools/typespec-ts`) now produces a single unified (Azure modular) output. In #4750 we collapsed the redundant `:azure-modular` / `:modular` npm script suffixes and pass-through aliases. The npm scripts are now clean, but the vestigial `modular` naming still survives at the vitest-project and folder level, where it's no longer meaningful.

## What's left to rename

These were intentionally left out of #4750 because they're config-/filesystem-level names that the contributor docs deliberately reference (`.github/instructions/typespec-ts.instructions.md`, `packages/typespec-ts/CONTRIBUTING.md`), so renaming them is a larger, more invasive change:

- **vitest projects** (`packages/typespec-ts/vitest.config.ts`):
- `integration-azure-modular` → e.g. `integration` (or `integration-azure`)
- `unit-modular` → e.g. `unit`
- **test folders:**
- `test/azure-modular-integration/` → e.g. `test/integration/`
- `test/modular-unit/` → e.g. `test/unit/`

## References to update in lockstep

- `packages/typespec-ts/vitest.config.ts` (project `name` + `include` globs)
- `packages/typespec-ts/package.json` scripts that pass `--project integration-azure-modular` / `--project unit-modular` (`integration-test:alone`, `unit-test`)
- `test/commands/*` that emit into `test/azure-modular-integration/generated/...` (e.g. `gen-spector.js`, `run.ts`)
- `.github/instructions/typespec-ts.instructions.md` (Test layout section + pipeline notes)
- `packages/typespec-ts/CONTRIBUTING.md` (test-suites table + "add an integration test case" walkthrough)
- The tracked generated baselines live under `test/azure-modular-integration/generated/**` — moving the folder will touch a large number of tracked `src/index.d.ts` files, so coordinate the move (prefer `git mv`) to keep the diff reviewable and `check:tree` green.

## Notes

- Naming is a judgment call — pick names that read well now that "modular" / "azure vs standard" distinctions no longer exist. Suggested: drop `modular` and keep `azure` only where it still disambiguates branded output.
- This is a pure rename/cleanup; no change to generated output or test behavior is expected.

Follow-up to #4750.

Start with packages/typespec-ts/vitest.config.ts and the package.json scripts to map the project names and include globs. Then update the test/commands references, contributor documentation, and tracked generated baselines using git mv where appropriate. Run the integration-test:alone and unit-test scripts plus check:tree; done means no vestigial modular names remain and generated output and test behavior are unchanged.

Written by the indexing model from the issue text.