Address feedback: enforce Azure DLL public key token at bind time under STRONG_NAME_SIGNING

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Description

When STRONG_NAME_SIGNING is enabled, the previous approach loaded the Azure extension assembly by simple name first, then checked the public key token post-load. This meant a malicious assembly with the same simple name could execute module initializers before the check ran.

Fix: Under #if STRONG_NAME_SIGNING, build a fully-qualified AssemblyName with the expected public key token set via SetPublicKeyToken before calling Assembly.Load. The CLR then enforces the token at bind time — the wrong assembly is never loaded.

#if STRONG_NAME_SIGNING
var qualifiedName = new AssemblyName(assemblyName);
qualifiedName.SetPublicKeyToken([0x23, 0xec, 0x7f, 0xc2, 0xd6, 0xea, 0xa4, 0xa5]);
var assembly = Assembly.Load(qualifiedName);  // runtime enforces token during binding
#else
var assembly = Assembly.Load(assemblyName);
#endif

Token mismatch or missing assembly causes Assembly.Load to throw FileNotFoundException, already handled by the existing catch block. The redundant post-load token check block has been removed.

Without STRONG_NAME_SIGNING, behavior is unchanged — simple name load as before.

Issues

Addresses review feedback on #3978.

Testing

The existing exception handling covers the token-mismatch failure path (runtime throws FileNotFoundException on token mismatch, caught by the existing handler). No new automated tests added; the behavior under STRONG_NAME_SIGNING is exercised by the existing AKV provider load path in the build that enables signing.

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[Copilot]

Pull request overview

This PR addresses a security vulnerability in how the Azure extension assembly is loaded under strong-name signing. The previous approach in PR #3978 loaded the assembly by simple name first, then validated the public key token post-load. This created a window where a malicious assembly with the same simple name could execute module initializers before the validation occurred.

Changes:

• Moved public key token enforcement to bind time by calling SetPublicKeyToken before Assembly.Load under STRONG_NAME_SIGNING
• Removed redundant post-load token validation block (18 lines)
• Leverages existing exception handling (FileNotFoundException is already caught) to handle token mismatch failures