feat(all): reportMergingTool support single report

[frank-mupt]

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[quanru]

Main concern: the new reportFilePath?: string flow is only implemented in ReportMergingTool, but it is not wired through the report viewer yet.

1. mergeReports() now writes <script type="midscene_web_dump"> entries with an empty body when reportFilePath is missing. However, the viewer still treats every dump script as JSON and eagerly selects the first case on load. If the first case is skipped, dumps[0].get() falls back to a non-GroupedActionDump object and setGroupedDump() still reads dump.executions.length, which can crash the page before the new disabled-click guard has any chance to help.

2. Related to that, the all skipped case is still broken in the viewer path. The new unit tests only assert that the merged HTML contains the expected attributes, but they do not verify that apps/report can actually load and render those merged reports.

3. This also broadens the public type too much. Making reportFilePath optional for all ReportFileWithAttributes values allows invalid states such as passed + undefined reportFilePath. A narrower model would be safer here, for example a discriminated union for skipped/non-viewable cases instead of weakening the whole API.

Minor point: removing the old <= 1 guard also changes the 0 reports behavior. mergeReports() now creates and returns an empty template-only report when nothing was appended. I think supporting 1 report makes sense, but 0 report should probably still return null or throw explicitly.