fix(flow): pass child input from selected transition

[gbaudrit]

Summary

Complete the visual Flow-call authoring path with one explicit checkbox that passes the output carried by the transition actually taken into a child Flow. Closes #278 and extends the composable orchestration epic #256.

Changes

• Add a localized Pass incoming transition output checkbox to the generic Flow card.
• Persist transition passthrough as ${transition.output} and restore the checked state from the draft definition.
• Resolve ${transition.output} from the selected incoming transition at runtime, including durable child-Flow resume.
• Accept complete structured transition output during published child-Flow contract validation.
• Verify the Agent -> Flow path by asserting that structured agent output becomes the child Flow Run input.
• Keep existing explicit property mapping behavior unchanged and outside this increment.

Validation

• dotnet restore Agentstration.slnx succeeded after synchronizing with current main through the Compose and invoke reusable Flows across triggers, agents, APIs, MCP, and other Flows #256 integration branch.
• dotnet build Agentstration.slnx --configuration Release --no-restore succeeded with 0 warnings and 0 errors.
• dotnet test Agentstration.slnx --configuration Release --no-build succeeded: 831 total, 827 passed, 4 skipped optional integrations, 0 failed.
• Focused Flow-call application tests succeeded: 6 passed, 0 failed.
• Focused Flow Designer tests succeeded: 26 passed, 0 failed.
• GitHub build check passed on the synchronized branch.
• Desktop and mobile executable smoke testing was not run; the visual interaction is covered by the Flow Designer component test.

Breaking changes

None.