SEP-2243 Clarifications

[mikekistler]

This issue will track several minor clarifications and corrections to SEP-2243 HTTP Standardization.

• Restrict values for x-mcp-header to match HTTP field-name token syntax
• Specify behavior for header value beginning with =?base64? that signals base64 encoding
• Add a note about best practice for comparing numeric values between header and body
• Add a note suggesting client behavior when it does not have the tool input schema

[mikekistler]

Other possible clarifications based on comments on the SEP-2243 PR after it was merged:

• x-mcp-header validation scope: clarify whether validation/filtering is Streamable HTTP-only and should not affect STDIO behavior.

• Nested vs top-level x-mcp-header: clarify whether annotations are allowed on nested properties or only top-level tool arguments.

• Base64 marker casing: clarify whether =?base64?...?= matching is case-sensitive or case-insensitive.

• Who sets mirrored values / dynamic vs static headers: clarify that this mechanism is for dynamic per-call values and distinguish from static configured headers.

• Authorization semantics for intermediaries: clarify that authorization context must be applied before acting on user-scoped mirrored headers.

• Protocol-version gating for intermediaries: clarify behavior for older/newer client-server combinations when intermediaries rely on mirrored headers.