[2212] Refactoring migration scheduling and validations

[meghansh-pf9]

What this PR does / why we need it

A VM with no matching OpenStack flavor used to abort the whole MigrationPlan — every VM behind it in the list never got a ConfigMap or Job, and the plan retried forever on the same VM.

Now the failure is contained to that VM:

• Flavors are resolved in pre-flight validation, once per plan instead of once per VM (and lazily, so operator-pinned plans still never call Nova).
• ErrNoSuitableFlavor sentinel + errors.Is so only an unschedulable shape is treated as a skip; auth/connectivity errors still requeue. Replaces a strings.Contains check.
• The unschedulable VM gets a Migration in ValidationFailed with an actionable reason, so it stays visible in the UI (no UI changes needed).
• processMigrationPhases accumulates per-VM failures instead of returning on the first one; the plan is marked PodFailed only once nothing is still running, so post-migration still runs for the VMs that succeeded.

Which issue(s) this PR fixes

fixes #2212

Testing Done

[github-actions]

✅ Security Vulnerability Summary

No change in security posture

📊 Overall Changes

Metric	Count
Total Added	0
Total Fixed	0
Net Change	0

🔍 Detailed Breakdown

📦 Gosec (Static Analysis)

Current	Baseline	Added	Fixed	Method
0	0	0	0	artifact

📦 Trivy (Dependency Scan)

Current	Baseline	Added	Fixed	Method
78	78	0	0	artifact

📋 Baseline Methods

• 📦 artifact: Used stored report from main branch
• 🔄 live_scan: Scanned base branch in real-time
• ⚠️ no_baseline: No baseline available (all vulnerabilities treated as new)

---

Only HIGH and CRITICAL severity vulnerabilities are tracked
Baseline: 3348a95cafb80aaa932c92e2d1ed418410962787

[github-actions]

✅ Security Vulnerability Summary

No change in security posture

📊 Overall Changes

Metric	Count
Total Added	0
Total Fixed	0
Net Change	0

🔍 Detailed Breakdown

📦 Gosec (Static Analysis)

Current	Baseline	Added	Fixed	Method
0	0	0	0	artifact

📦 Trivy (Dependency Scan)

Current	Baseline	Added	Fixed	Method
78	78	0	0	artifact

📋 Baseline Methods

• 📦 artifact: Used stored report from main branch
• 🔄 live_scan: Scanned base branch in real-time
• ⚠️ no_baseline: No baseline available (all vulnerabilities treated as new)

---

Only HIGH and CRITICAL severity vulnerabilities are tracked
Baseline: 3348a95cafb80aaa932c92e2d1ed418410962787