[Kehrlann]

• Transports need to list a context's entries and combine contexts, for instance to add the decoded Mcp-Param-* headers to a context built by a user-provided extractor.
• This introduces a way to merge McpTransportContext. To do so, it introduces a way to read entries from the contexts, by key-value.
• This change is non-breaking but custom implementations of McpTransportContext used in the modern server MUST implement stream().

The implementation is inspired from Reactor's Context.

[chemicL]

#1288 as an alternative solution to the need.

Reasons:

• We don't have to create a generic Context-like implementation here.
• We don't need to iterate at this time as the intended usage is to grab a specific key's value.
• For the most common case we can just merge two maps, we don't need streaming/iterating over the contents.
• In case of user implementations of the McpTransportContext implementation we can provide an overlay instead of iterating, too.

Thus, we can avoid the breaking change and just have a default method with a working implementation.