Escape aliases in metrics view search fallback query (value is reserved in Druid SQL)

[nishantmonu51]

• The global dimension search (dimension_search feature flag) fails on Druid with Incorrect syntax near the keyword 'AS'. The SQL fallback for Executor.Search wraps each per-dimension query as SELECT '<dim>' AS dimension, "<dim>" AS value FROM (...), and VALUE is a reserved keyword in Druid's Calcite parser. The error columns reported in the thread (52 and 60) match the position of AS value for the account_name and line_item_alt_id dimensions exactly.
• The fallback runs whenever the native Druid search cannot be used (filter not translatable to a native filter, missing time range, or unnested dimension), so it has to work on Druid too.
• Escape both aliases via the dialect so Druid gets AS "dimension" and AS "value". No change in behavior for DuckDB or ClickHouse, which do not reserve value.
• Adds executor_search_test.go, the first test covering the search fallback path (multiple dimensions, with and without a where clause and time range). Note it runs on DuckDB and so cannot reproduce the Druid keyword failure itself.

Closes APP-935

Slack: https://rilldata.slack.com/archives/C02T907FEUB/p1789483677280479

Checklist:

• Covered by tests
• Ran it and it works as intended
• Reviewed the diff before requesting a review
• Checked for unhandled edge cases
• Linked the issues it closes
• Checked if the docs need to be updated. If so, create a separate Linear DOCS issue
• Intend to cherry-pick into the release branch
• I'm proud of this work!

[Copilot]

Copilot was unable to review this pull request because the user who requested the review has reached their quota limit.

[Copilot]

🟡 Changes recommended

Add a Druid-dialect regression assertion for fallback alias escaping.

Get a fresh assessment by requesting another Copilot review.

Review details

• Files reviewed: 8/8 changed files
• Comments generated: 1
• Review effort level: Lite