fix(migtd): reject zero-progress pre-session writes

[haitaohuang]

Rationale

send_pre_session_data retries until the full buffer is written, but an AsyncWrite implementation may return Ok(0) without making progress. Treat that result as a logged MigrationResult::NetworkError so the migration pre-session path cannot spin indefinitely.

This matches the fail-closed behavior already used by the SPDM exact-I/O path in #1022.

Validation

• cargo fmt --check
• Full non-EMU CI gauntlet: preparation, format/check/Clippy, deny, library, and all 32 main image builds
• Current Intel integration-emu.yml matrix, all 14 scenarios run sequentially:
  • Build and Test (Skip RA)
  • Policy v2 with Mock Report
  • Policy v2 with Mock Report and IGVM Attest
  • Rebind Prepare (Skip RA)
  • Rebind Prepare (Mock Report)
  • SPDM Migration (Skip RA)
  • SPDM Policy v2 with Mock Report
  • SPDM Rebind Prepare (Skip RA)
  • Mock Quote Retry, including retry-log assertions
  • Policy v2 Key Rotation (Mock Report)
  • Policy v2 Key Rotation with IGVM Attest
  • SPDM Policy v2 Key Rotation (Mock Report)
  • Policy v2 Policy + TCB Mapping Leaf Rotation (Mock Report)
  • Policy v2 All Three Chains Rotation (Mock Report)