fix: fail tools-list when tools array is empty

[lux999]

An empty array ([]) is truthy in TypeScript, so the tools-list check incorrectly passed even when no tools were returned. Also refactor the nested if/else into a flat if/else-if chain for readability.
Please see Additional Context for the decision.

Motivation and Context

The tools-list scenario incorrectly reported success when a server returned an empty tools array ([]). An empty array is truthy in TypeScript, so !result.tools did not catch it, and forEach on [] was a no-op — resulting in zero errors and a passing test. This masked real failures where a server's initialize handshake failed and it returned no tools.

How Has This Been Tested?

Verified locally with vitest — all 84 tests pass. Confirmed manually that a server returning [] for tools/list now produces a failure, while a server returning valid tools still passes.

Breaking Changes

Servers with legitimately zero tools will now fail the tools-list scenario. These servers should add tools-list to their expected-failures baseline.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

We understand there are two distinct cases where a server returns [] for tools/list:

1. Intentional — the server is healthy but has no tools configured.
2. Error-induced — the server's initialize handshake failed and it returned [] as a result, masking a real failure.

Three approaches to distinguish these cases were considered:

1. Config parameter (e.g. --min-tools 1 passed to the conformance runner) — explicit but adds complexity to the CLI and every baseline file that needs to opt in.
2. Separate scenario (e.g. tools-list-nonempty that servers opt into) — more surgical and avoids breaking existing baselines, but duplicates logic from the existing tools-list scenario.
3. Fail all empty arrays in tools-list — any server returning [] fails. A server with no tools simply adds tools-list to its expected-failures baseline, consistent with the existing convention.

We went with option 3 as the right balance of correctness and simplicity. Raising as a draft for team discussion on the edge-case policy before merging.