Refactor: Extract shared Foundry project selection logic into helper function

[v1212]

Background

PR #8292 introduced a two-step Foundry project selection flow in both init.go and init_from_code.go. The two files now share ~70 lines of near-identical logic:

ensureSubscription → selectFoundryProject → nil check → setEnvValue("USE_EXISTING_AI_PROJECT") → fallback to "create new"

Problem

Future fixes may drift between the two files if only one is updated.

Proposed Solution

Extract a shared helper function (e.g. promptOrSelectFoundryProject(...)) that encapsulates the common sequence, and have both callers use it.

Files

• cli/azd/extensions/azure.ai.agents/internal/cmd/init.go (~L1182-1250)
• cli/azd/extensions/azure.ai.agents/internal/cmd/init_from_code.go (~L520-590)

Priority

P3 — non-blocking polish, suggested by @wbreza in PR #8292 review.