fix(server): name a file 'explores' actually lists in the tile warning's fix

[jswir]

Follow-up to #1206, from Nathan's re-review of lintTilesAgainstSurface.

The index.malloy case he raised can't be reached. With no explores key, the surface is index.malloy alone. Dashboards only come from dashboards/*.malloy, and none of those are on that surface, so every dashboard is held back before the tile check runs. The held-back warning already branches on surfaceIsIndexModel(). The test now pins this: with no key, the dashboard isn't served and there is no tile warning. If a later change to the gate lets one through, that test fails.

A real problem in the same text. The fix always said "import it into a listed file such as index.malloy". It went wrong like this:

1. explores lists orders.malloy and dashboards/dash.malloy.
2. A root index.malloy exists but isn't listed.
3. The author imports the source into index.malloy.
4. Nothing changes, because an unlisted file publishes nothing.

Changes:

• The example is now the first model explores actually lists, not counting dashboard files. If there isn't one, the text says "a file you list there".
• The first half now says to add the declaring file "keeping the entries already there", so it can't be read as replacing the key.
• A comment explains why this remedy doesn't branch on surfaceIsIndexModel().

Tested: query_boundary.spec.ts passes (26 tests), and the old wording fails the new assertions.

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