Fix the remaining culture-sensitive lookups (Esent type names, unit symbols, property sets)

[oguzozshn]

Follow-up to #672, taking you up on the offer to look at the remaining culture-sensitive code.
Three separate defects, one commit each, each with a test that fails without its fix.

1. Express type name lookup in the Esent store

EntityCollection.OfType(string) already upper-cases with the invariant culture before hitting the
metadata dictionary, which is keyed on invariant upper case. PersistedEntityInstanceCache.OfType
used ToUpper(), so the same public API returned results or nothing depending on which store backed
the model. Under tr-TR, "IfcBuilding" upper-cases to "IFCBUİLDİNG" and never matches.

The test deliberately queries IfcBuilding rather than IfcWall — a type name only trips the
Turkish casing rules if it contains a lower case i, and IfcWall does not.

2. Imperial unit symbols

IfcConversionBasedUnit.Symbol matched the unit name against "FEET", "FOOT" and "INCH" after
upper-casing it. Under tr-TR, "inch" becomes "İNCH", which does not contain "INCH", so the
symbol fell through to the raw name instead of "in". Replaced with an ordinal case-insensitive
IndexOf, which also drops the intermediate allocation. All three schemas.

3. Property set lookups

GetPropertySet and GetElementQuantity implemented their caseSensitive: false option with
culture-sensitive comparisons — string.Compare(a, b, ignoreCase), one explicit
CurrentCultureIgnoreCase, and a pair of ToLower() calls. Property set names are IFC identifiers
rather than prose, so these now use OrdinalIgnoreCase, and the case-sensitive path Ordinal.
Ifc4x3 has no equivalent helpers.

Deliberately left alone

I went through every remaining ToUpper() / ToLower() in the tree and did not touch:

• ~2650 occurrences in the generated schema projects — all of the form
  GetType().Name.ToUpper() inside XbimParserException messages. They only affect the text of an
  error, and they come from the code generator rather than these files.
• XbimXmlReader3's switch (pt.ToString().ToLower()) — its only labels are "string" and
  "boolean"; neither contains a lower case i, so the casing cannot change the outcome.
• The Esent enum write in IPersistEntityExtensions — I had changed this, then reverted it after
  confirming every IFC enum member is already upper case ASCII, which makes ToUpper() a no-op
  there. It seemed better to leave the diff honest than to pad it.

Happy to fold any of these in if you would rather have the codebase uniform.

Verification

On a tr-TR machine: Xbim.Essentials.Tests 500/500 on both net10.0 and net48,
Xbim.Essentials.NetCore.Tests 94/94. Each fix was also checked by stashing it and confirming its
test fails.

Refs #670.