Fix McpHeaderEncoder.DecodeValue throwing on the degenerate base64 wrapper

[latent-9]

McpHeaderEncoder.DecodeValue throws ArgumentOutOfRangeException on the input "=?base64?=" instead of returning it unchanged, which contradicts its own contract ("returns the original value" when the value is not a valid base64 wrapper) and its never-throw test suite.

The wrapper uses prefix "=?base64?" (ends with ?) and suffix "?=" (starts with ?), which overlap on the shared ?. So "=?base64?=" (length 10) satisfies both StartsWith(prefix) and EndsWith(suffix). The inner length is then computed as Length - Prefix.Length - Suffix.Length = 10 - 9 - 2 = -1, and Substring(9, -1) throws. Because that Substring is outside the try, the exception propagates.

This is reachable from a client-supplied header (for example Mcp-Name: =?base64?=): every character is a valid header character, so it passes validation and reaches DecodeValue, turning an intended graceful rejection into an unhandled exception.

Fix: guard the wrapper branch with a minimum-length check, so a value too short to hold any content falls through to the literal return (the same behavior as DecodeValue_MissingSuffix_ReturnsLiteralValue). Added a regression test.