fix(core): export writes the openqa_overview results into the report document

[mimi1vx]

Problem

export reads openqa().overview and authors it into testing.openqa.extra, but nothing ever set that field. openqa_overview printed its result and dropped it; only openqa_overview --export authored it. Running openqa_overview and then export left the single incidents, aggregated updates and build checks out of the document, for every workflow (Auto, Kernel, Manual).

Change

• openqa_overview saves its full (not row-limited) result on the report, with or without --export.
• export writes the saved result. When none is saved it fetches one with openqa_overview's defaults (5 days, group core, aggregated included, URLs from config).
• A failed fetch prints WARNING: openQA overview unavailable: <err> and the export continues without those rows. This matches the Manual workflow's existing dashboard fetch.
• The fetch moved into fetch_overview so both commands share it. openqa_overview output is unchanged.
• The --export help no longer refers to the removed text log. This changes the MCP slim schema description for openqa_overview.export.

Tests

• overview_then_export_authors_testing_openqa_extra: runs both commands in sequence. Red before the fix.
• export_fetches_overview_when_none_saved, export_reuses_saved_overview (the overview endpoints get zero requests), export_overview_fetch_failure_still_exports, overview_is_saved_on_the_report.
• Existing export tests seed an empty overview so they never reach production URLs.

I observed the new tests red by removing the save assignment and the saved-result guard.

Notes

• openqa_overview now treats an empty single_incidents as "no openQA builds yet". That is exact today because single_incidents returns one row per version.
• A malformed openQA client.conf now shows the OpenQA: header before its error, because the client is built inside fetch_overview.

Local gate green: fmt, clippy --all-features -D warnings, rustdoc -D warnings, cargo test --workspace, cargo test -p mtui-mcp -F mcp, and both feature builds.