feat(google_adk): add ADK-115, ADK-116 tool description quality rules

[bradAGI]

Ports the CSDK-017/018 description-quality pair (merged in #40) to the Google ADK. ADK-001 only checks that a docstring exists, so a tool whose docstring reads """TODO: describe this tool.""" or """Gets data.""" passes today while giving the model no selection signal.

Two ADK-specific points:

• The docstring does double duty. The ADK parses it for the per-parameter descriptions that accompany the generated schema, so a placeholder or one-clause docstring strips argument-level guidance at the same time as tool-level guidance. Both fixes ask for documented parameters, not just a longer summary.
• In an agent tree, mis-selection doesn't stay local. A tool picked wrongly inside one branch produces output the following agents treat as established fact — the error gets laundered into shared session state rather than surfacing where it began.

ADK-116 also names the mitigation people would reach for and why it isn't one: ADK-102 and ADK-107's before_tool_callback can block a call it recognizes as wrong, but it can't supply the judgment the description was meant to provide. A callback doesn't compensate for a thin description.

Numbered ADK-115/116 to leave room for ADK-111 in the open PR #51 and the IDs used by my other open PRs (#79, #86).

Verification — engine built at main:

$ trustabl rules validate .
OK: 85 rule pack(s), 208 rule(s) valid under rule schema version 14

Fire (one FunctionTool-wrapped function with """TODO: describe this tool.""", one with """Gets data."""): ADK-115, ADK-116, ADK-201
Silent (full docstring with an Args: section): ADK-201

ADK-116 pairs description_length_lt: 40 with has_docstring: true so it doesn't double-report against ADK-001 on an empty docstring, same as CSDK-018.

No new predicates, so no schema_version bump.