Description

Closes the discover -> pull -> query loop in the MCP server so an agent can find providers in the registry, pull them, and query without a human pre-pulling from a shell. Bundles two read-path fixes the new tools depend on.

• Fix 1 - empty result sets reported as extraction failures. extractQueryResults returned (rv, ok && len(rv) > 0), so a zero-row result was mapped to failed to extract query results. Now returns (rv, ok) and treats a nil SQLResult (what PrepareResultSet emits for an empty RowMap) as a zero-row result rather than an error. rv is initialised as an empty slice so it survives QueryResultDTO.Rows schema validation downstream. Repro: fresh approot, no providers pulled, list_providers -> failed to extract query results.
• Fix 2 - literal/expression columns rendered as Go nullable wrappers. RenderTable.dataRow and RenderKV formatted with %v, assuming scalars; literal columns carry a pointer-to-nullable-wrapper so cells rendered as &{ok true}. Added an unwrap helper (handles value + pointer forms of sql.NullString, NullBool, NullInt64, NullInt32, NullFloat64, NullByte, NullTime, generic sql.Null[T]) applied before formatting in both renderers. Invalid wrappers render as empty cells. Repro: SELECT 1 as n, 'ok' as status -> | &{ok true} | &{1 true} |. Provider-backed columns were unaffected (unwrapped upstream).
• Feature 1 - list_registry tool. Lets an agent see providers available in the registry to pull, distinct from list_providers (already-pulled). Optional provider arg lists versions for that provider; omitted lists all. New interrogator method GetRegistryList(provider string) builds REGISTRY LIST [<provider>]; backend ListRegistry routes through runPreprocessedQueryJSON. Table renderer. Registered with selectGate semantics (read-only, allowed in all modes).
• Feature 2 - pull_provider tool. Lets an agent install a provider into the approot cache so subsequent queries resolve. Required provider, optional version. New interrogator method GetRegistryPull(provider, version string) builds REGISTRY PULL <provider> [<version>]; backend PullProvider routes through execQuery. KV renderer ({messages, timestamp}). Allowed in all modes: pulling fetches remote content and writes local state but does not touch any cloud control/data plane, so it is classified QueryClassSelect, not a cloud mutation. If a human-in-the-loop is wanted later, this would become a new registry_pull query class gated like lifecycle.

Forbidden-character guard (space, ;, \) mirrors the CLI registry command on both new tools so user-supplied identifiers cannot smuggle a second statement.

Why CI didn't catch the read-path fixes

No test asserted on an empty result set, and query/metadata tests assert on structuredContent JSON + row counts, never on rendered cell values; every test SELECT hit a real provider table, so no test ran the literal/no-FROM shape that exposes the wrapper.

Robot scenario notes

• MCP HTTP Empty Result Renders Cleanly asserts on the wire shape "rows":[], because the mcp_client's formatToolResult prefers StructuredContent (the JSON DTO) over the renderer's **no results** text. The renderer's empty branch is still covered by TestRenderTable_Empty / TestRenderKV_Empty.
• MCP HTTP List Registry Returns Available Providers pins the any-sdk refusal 'registry list' is meaningless in local mode, because the MCP servers in this suite use a file:// registry URL (per _get_registry_no_verify), and any-sdk's ListAllAvailableProviders deliberately refuses for local-mode registries. That refusal is the contract for this config; the tool surface and round-trip are covered end-to-end by the Go unit tests against an HTTP-shaped backend.

Type of change

• Bug fix (non-breaking change to fix a bug).
• Feature (non-breaking change to add functionality).
• Breaking change.
• Other (eg: documentation change).

Issues referenced.

Closes #661

Evidence

• Unit tests: TestRenderTable_UnwrapsNullableWrappers, TestRenderKV_UnwrapsNullableWrappers, TestRender_InvalidNullableRendersAsEmpty, TestTool_ListRegistry_RendersTableAndForwardsProvider, TestTool_ListRegistry_AllowedInReadOnly, TestTool_PullProvider_RendersKVAndForwardsArgs, TestTool_PullProvider_AllowedInReadOnly, plus TestRegistration_EnabledToolsFilters extended to assert the two new tool names are filterable.
• Robot scenarios added under test/robot/functional/mcp.robot:
  • MCP HTTP Empty Result Renders Cleanly
  • MCP HTTP Literal Select Renders Unwrapped Scalars
  • MCP HTTP List Registry Returns Available Providers
  • MCP HTTP Pull Provider Installs Known Provider
• All 33 previously-passing MCP robot scenarios remain green in the run that motivated the final assertion tightening.

Checklist:

• A full round of testing has been completed, and there are no test failures as a result of these changes.
• The changes are covered with functional and/or integration robot testing.
• The changes work on all supported platforms.
• Unit tests pass locally, as per the developer guide.
• Robot tests pass locally, as per the developer guide.
• Linter passes locally, as per the developer guide.

Variations

None.

Tech Debt

Zero technical debt accrued. The read-path fixes are additive on the (rv, ok) contract and provider-backed output is unchanged. The new tools route through the existing interrogator + backend + gate pattern with no new abstractions. The only open design decision (gating class for pull_provider) is resolved in-line: classified as QueryClassSelect because it writes only local cache state, with a clear migration path to a dedicated registry_pull class if a human-in-the-loop is ever wanted.