test(rules): mirror and cover MCP-025, MCP-026

[bradAGI]

Engine half of a coordinated pair. Rules half: trustabl/agent-reliability-rules#88, on a branch of the same name, so the rules-sync job resolves the matching pack rather than main. Neither half should merge alone — check-rules-sync.sh fails if they do.

What the pair adds

Ports the CSDK-017/018 description-quality pair to MCP. MCP-001 only checks that a docstring exists, so """TODO: describe this tool.""" and """Gets data.""" pass today.

A placeholder costs more across a protocol boundary than inside an in-process SDK, for three reasons the rule text makes explicit: the consumer has no fallback context (it can't read the source or neighboring tools); the server's authors never observe the mis-selection, since it surfaces in someone else's client session; and a client typically connects several servers at once, so the model picks across all of them from these strings alone.

What this PR does

1. Mirrors mcp/tool_definition.yaml into testdata/rules-fixture/.
2. Adds cases to policyRuleCases, as TestPolicyRules_AllRulesCovered requires.

case	expectation
MCP-025 — """TODO: describe this tool."""	fires
MCP-025 — real description	silent
MCP-026 — """Gets data."""	fires
MCP-026 — full sentence	silent
MCP-026 — no docstring at all	silent

Five cases rather than four. MCP-026 pairs description_length_lt: 40 with has_docstring: true so an absent docstring stays MCP-001's finding rather than double-reporting. The fifth case pins that guard.

Verification

$ RULES_REPO=../trustabl-rules scripts/check-rules-sync.sh
rules fixture is in sync with production (86 files compared)

$ go vet ./internal/rules/
$ go test ./internal/rules/
ok  	github.com/trustabl/trustabl/internal/rules

[jhumel-code]

Thanks @bradAGI. This is a well put together pair. The fixture copy of mcp/tool_definition.yaml is byte identical to the production half in trustabl/agent-reliability-rules#88, both rules reuse predicates that already exist on main (has_description_text, has_docstring, description_length_lt), and MCP-025/MCP-026 land in a free slot (MCP-023 and MCP-024 are claimed by other open PRs, but nothing here collides).

The five cases are the right shape. Reading them against the match expressions: the placeholder case trips the todo needle, the stub case is 10 characters against the 40 threshold, and the absent-docstring case genuinely pins the has_docstring: true guard rather than passing vacuously, which is the reason that guard is there.

Two small things. The diff also removes an unrelated blank line at the end of policyRepoRuleCases; please drop that hunk, since this file is heavily contended right now and every extra hunk is conflict surface. Separately, a note for the rationale doc rather than the YAML: has_description_text is a case-insensitive substring match, so a task-management server with "Add a todo item to the list." will fire MCP-025. Worth naming under known gaps.

Both rules still need a rulebook rationale doc before they ship, and we can handle or pair on that. Merge this together with trustabl/agent-reliability-rules#88, since rules-sync fails if either half lands alone. Happy to take it once the stray hunk is gone.