fix: expose GST duty head on ledgers

[benzene73]

Summary

• expose Tally's ledger-level GSTDUTYHEAD as Ledger.GSTDutyHead
• use the existing GSTTaxType enum for CGST, SGST/UTGST, IGST, cess, and UT tax values
• verify the generated ledger request fetches GSTDUTYHEAD
• cover deserialization with an IGST ledger fixture

Why

Tally's documented ledger schema stores the specific GST tax head in GSTDutyHead: https://help.tallysolutions.com/article/DeveloperReference/tdlreference/gst_schema.htm/

The current model exposes GSTTaxType through the GSTTYPE XML element. Because v3 generates a TDL report from declared model fields, ordinary ledger reads currently omit GSTDUTYHEAD. In a live TallyPrime 7 read, the SDK response for an IGST ledger contained <TAXTYPE>GST</TAXTYPE> but no specific head; explicitly fetching $GSTDutyHead returned IGST.

This adds the documented field without changing or removing the existing GSTTaxType property.

Test

dotnet test src/Tests/TallyConnector.XmlTests/TallyConnector.XmlTests.csproj --filter "FullyQualifiedName~LedgerDeserializationTests"
Passed: 8, Failed: 0

[saivineeth100]

I am going to achieve this repo

my initial though of separate services, model per version made this library hard to maintain
so, I started fresh another repo using unified domain model with differed transport models based on version
since its build with AI before releasing public, I am testing very thoroughly
and there are lot of issues especially dynamic xml based on model
select statement support
if you are interested, I can add you privately