Update input content types

[danmichaeljones]

• Updates PreExtractedContent to include Topic field to match backend changes
• Adds ConversationContent model to allow passing optional metadata fields with conversations (e.g., to support tool calls)
• Adds matching StringContent model for future proofing (e.g. adding optional created_at timestamps later) / consistency with other content types

[orca-security-eu]

Orca Security Scan Summary

Status	Check	Issues by priority
Passed	Infrastructure as Code	0   0   0   0	View in Orca
Passed	SAST	0   0   0   0	View in Orca
Passed	Secrets	0   0   0   0	View in Orca
Passed	Vulnerabilities	0   0   0   0	View in Orca