feat(client): send MCP standard POST headers

[AndreKalberer]

Problem

SEP-2243 requires Streamable HTTP clients to mirror applicable JSON-RPC request metadata into standard HTTP headers. The Kotlin client currently sends Mcp-Protocol-Version and session metadata but does not emit Mcp-Method or Mcp-Name.

Solution

• Add Mcp-Method for JSON-RPC requests and notifications.
• Add Mcp-Name from a string params.name, falling back to a string params.uri.
• Omit both headers for JSON-RPC responses.
• Add focused transport tests for name-, URI-, notification-, and response-based behavior.

Validation

• .\gradlew.bat :kotlin-sdk-client:jvmTest --tests "io.modelcontextprotocol.kotlin.sdk.client.streamable.http.StreamableHttpClientTransportTest"
• .\gradlew.bat :kotlin-sdk-client:ktlintCheck :kotlin-sdk-client:detekt :kotlin-sdk-client:apiCheck
• git diff --check

Closes #787

AI assistance was used while preparing this PR. I reviewed the final change, ran the validation listed above, and take responsibility for it.

[devcrocod]

I think this PR does not follow the value encoding specified in the spec. Please take a look: https://modelcontextprotocol.io/specification/draft/basic/transports/streamable-http#value-encoding

[AndreKalberer]

Addressed in commit bb8ff1a. Mcp-Name now uses the specified Base64 sentinel encoding for non-ASCII or control characters, leading or trailing whitespace, and sentinel-shaped values. I added exact regression coverage for each case and reran the focused JVM tests, ktlint, detekt, API checks, and multiplatform compilation; all pass. AI assistance was used while preparing this response; I reviewed and take responsibility for the change.