fix(mcp): prevent get_bank from creating banks

[Sanderhoff-alt]

Summary

• Treat the core MCP get_bank tool as read-only.
• Look up bank profiles with create_if_missing=False in both single-bank and multi-bank MCP modes.
• Return a not-found error when the bank is missing instead of creating it.
• Add MCP regression tests for missing banks and for the non-creating lookup parameter.

Why

get_bank is a read tool: it returns bank metadata such as name, disposition, and mission. Before this change, it called get_bank_profile() with the default behavior, which can create a missing bank as a side effect.

That makes a read-only MCP call mutate persistent state when a client passes a stale or mistyped bank id. It also makes lifecycle and authorization semantics harder to reason about, because get_bank effectively becomes a hidden create path.

Creation should stay on explicit write/create surfaces such as create_bank. This change keeps get_bank read-only while preserving its existing response shape for successful reads.

Test

• uv run pytest tests/test_mcp_tools.py::TestTagsAndBankTools -q
• git diff --check