[28.x] E-Document import: vendor-matching telemetry and Posting Description fixes

[ventselartur]

Summary

Improvements to the E-Document purchase import flow (backport to 28.x):

• Vendor-match telemetry: emit E-Doc. Imp. Session Telemetry signals throughout the prepare-draft pipeline — Vendor Match Method (No / GLN / VAT Id / Service Participant / Name and Address / None), Vendor Info Present, Vendor Assignment Source (Already Assigned / Provider / History / None), and Vendor Matched By Name Not Address.
• Name-and-Address fallback fix: the result of FindVendorByNameAndAddress was previously discarded (trailing ;); it now actually assigns the matched vendor and exits.
• Posting Description fix: only copy Posting Description from the E-Document onto the Purchase Header when it is non-empty, and apply it after the default posting date logic so a blank value no longer overwrites the header.
• Remove the obsolete #if not CLEAN26 guard around the Microsoft.eServices.EDocument.Processing.Import using statement (now always required).

Work Item(s)

Fixes AB#639957
Fixes AB#639956