feat(tool/looker): add create_merge_query tool

[LacombeLouis]

Description

Adds a new Looker tool, looker-create-merge-query (registered as
create_merge_query), exposing Looker's
Merged Results through the
API (POST /merge_queries).

Impact: a query call can only target one explore, so today an agent that
needs fields from two explores has to run both queries and stitch the rows
together itself — losing Looker's join semantics and burning extra tool calls.
This tool performs the merge server-side, including across explores backed by
different databases.

Solution: the tool takes a source_queries array, one entry per explore.
Order is significant — the first entry is the primary query, and the results of
every later entry are joined onto it using that entry's merge_fields, matching
Looker's left-outer-join semantics. Each source query is created with
sdk.CreateQuery (idempotent, so an identical query is reused rather than
duplicated) and the resulting query ids are handed to sdk.CreateMergeQuery.
Merge-level pivots, sorts, limit, column_limit, total,
dynamic_fields and vis_config are supported.

Important API limitation

Looker has no endpoint that runs a merge query — only create and get. So the
tool returns the merge query id, its result_maker_id, the created source
queries (name, model, explore, query_id, slug) and a url that opens
the merged results in the Looker UI, rather than rows of data. This is stated
explicitly in the tool description and the docs page so the planner does not
expect data back and instead falls back to query per explore when it needs the
rows in-conversation.

Notable details

• Strict source-query keys. An unknown key inside a source query is
  rejected rather than silently dropped, so a typo like explores surfaces as an
  error instead of a query quietly missing a filter.
• Unquoted-parameter filters. Reuses
  lookercommon.EscapeUnquotedParameterFilters per source query, so LookML
  type: unquoted parameters behave exactly as they do in looker-query.
• Discoverability. Adds a note to the prebuilt query tool description
  pointing at create_merge_query. The shared parameter descriptions in
  lookercommon are deliberately left untouched — changing them forces edits
  across five separate integration-test expectation blocks.
• Annotations. Uses NewWriteAnnotations (non-destructive write), since the
  call does create objects in Looker but nothing user-visible or destructive.

Testing

• Unit tests: YAML config parse (success + failure), processSourceQuery (happy
  path, every optional key, 10 error cases), processSourceQueries (arity guard),
  asRowLimit and mergeURL.
• go test -race ./cmd/... ./internal/... passes.
• go vet and gofmt clean, including tests/looker.
• .ci/lint-docs-tool-page.sh passes (320 pages).
• Integration tests: tool config plus a RunToolGetTestByName manifest
  assertion (verified byte-for-byte against the generated manifest) and a
  RunToolInvokeParametersTest invoke case.

Reviewer notes

• The integration invoke test is unverified against a live instance. It
  merges two system__activity look queries joined on look.id and assumes
  look.id / look.title exist there. I had no Looker instance available, so
  this is the most likely thing to need adjusting.
• golangci-lint is not installed in my environment and was not run; CI will be
  the first real check.
• .ci/lint-docs-sample-filters.sh fails locally, but it reports every filter in
  every untouched file as invalid — a pre-existing local tooling problem, not
  something this branch causes.

PR Checklist

• Make sure you reviewed
  CONTRIBUTING.md
• Make sure to open an issue as a
  bug/issue
  before writing your code! That way we can discuss the change, evaluate
  designs, and agree on the general idea
• Ensure the tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)
• Make sure to add ! if this involve a breaking change (not a breaking change)

🛠️ Fixes #3756 🦕

🤖 Generated with Claude Code

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[gemini-code-assist]

Code Review

This pull request introduces a new tool, create_merge_query (type looker-create-merge-query), to the Looker integration. This tool enables merging query results from multiple explores, mimicking a SQL left outer join. The changes include the Go implementation of the tool, comprehensive unit and integration tests, updates to prebuilt configurations, and extensive documentation. No review comments were provided, so I have no feedback to offer on the review itself.

[duwenxin99]

Hi @drstrangelooker, this contributor is adding a new Looker tool. Could you take a look and see if this is what you want? Thank you!