[http-client-csharp] Fix trailing path separator for optional path parameter

[JoshLove-msft]

Problem

When a route ends with an optional path parameter, the / separator preceding that segment was emitted unconditionally, leaving a dangling trailing slash when the value is null.

For example, /certificates/{certificate-name}/{certificate-version} (with certificate-version optional) generated:

uri.AppendPath(certificateName, true);
uri.AppendPath("/", false);            // emitted unconditionally
if ((certificateVersion != null))
{
    uri.AppendPath(certificateVersion, true);
}

So when certificateVersion is null the wire URL becomes /certificates/{name}/ instead of /certificates/{name}. Services (e.g. Key Vault) and recorded test cassettes expect no trailing slash.

The existing shouldPrependWithPathSeparator logic only handled the case where the preceding literal does not end in /; it missed the case where the separator had already been written unconditionally.

Fix

In RestClientProvider.AddUriSegments, when the upcoming path parameter is optional and the preceding literal ends with /, defer that trailing / so it is written together with the parameter value inside the null check:

uri.AppendPath(certificateName, true);
if ((certificateVersion != null))
{
    uri.AppendPath("/", false);
    uri.AppendPath(certificateVersion, true);
}

Required path parameters are unaffected.

Tests

• Added TestBuildCreateRequestMethodWithOptionalPathParameter (+ golden file) covering the trailing optional-segment case.
• Updated two ServerTemplate* tests that relied on the test factory's default isRequired: false for what were intended to be normal required path parameters.
• Full Microsoft.TypeSpec.Generator.ClientModel suite passes (1440 tests).

Reported in Azure/azure-sdk-for-net#60162 (Bug A).

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