refactor: consolidate MCP initialize payload builder and protocol-version constant

[salimovartem]

Part of #111.

Goal

Eliminate the copy-paste drift between the two initialize response literals (stdio vs HTTP) by extracting a single payload builder and a named protocol-version constant.

Context

The initialize response — protocolVersion, capabilities, serverInfo — is declared twice with the same content: once in plugins/corezoid/mcp-server/mcp_server.go:303-318 for the stdio transport, and once in plugins/corezoid/mcp-server/mcp_http.go:288-303 for the Streamable HTTP transport. The string "2025-03-26" is hardcoded in 9 places across the repo (2 production, 7 tests) with no shared constant. Any change to the response shape has to be applied twice, and there is no build-level guarantee that the two paths stay in sync.

This refactor is a prerequisite for the two follow-up tasks in the same milestone (server/discover handler and UnsupportedProtocolVersionError), both of which need to return the same capabilities as initialize. Consolidating first keeps the follow-ups small and prevents introducing a third divergent copy of the payload.

The repo currently reports itself as legacy-only (spec 2025-03-26) and stays that way in this PR. Do not raise the version.

Scope

In scope:

• Introduce const mcpProtocolVersion = "2025-03-26" in plugins/corezoid/mcp-server/mcp_server.go, adjacent to the existing mcpServerVersion constant (line 52).
• Add a helper (suggested name buildInitializeResult()) in mcp_server.go that returns the shared map[string]interface{} result body used by initialize — protocolVersion, capabilities (with tools, resources, prompts), serverInfo.
• Replace the inline literals in both dispatchers (stdio at mcp_server.go:303-318, HTTP at mcp_http.go:288-303) with a call to the new helper.
• Switch all seven test-side occurrences of "2025-03-26" to reference the new constant (or, where tests must remain string-literal for readability, add a compile-time assertion / helper). Files: mcp_protocol_test.go (4 occurrences at lines 91, 122, 166, 211), mcp_http_test.go:129, mcp_http_session_test.go:191, mcp_http_session_test.go:382.

Out of scope:

• Adding server/discover (separate issue).
• Returning UnsupportedProtocolVersionError on unknown versions (separate issue).
• Advertising any MCP extensions (Tasks, Apps, ui) — none of them are supported yet.
• Raising the protocol version above 2025-03-26.
• Touching Mcp-Session-Id, elicitation, or any other transport-level behaviour.

Files / areas to touch

• plugins/corezoid/mcp-server/mcp_server.go — add mcpProtocolVersion constant and buildInitializeResult() helper; replace inline literal in the initialize case (line ~297 switch, ~303-318 payload).
• plugins/corezoid/mcp-server/mcp_http.go — replace inline literal in the initialize case (line ~272 switch, ~288-303 payload) with the shared helper.
• plugins/corezoid/mcp-server/mcp_protocol_test.go — update lines 91, 122, 166, 211 to use the constant.
• plugins/corezoid/mcp-server/mcp_http_test.go — update line 129.
• plugins/corezoid/mcp-server/mcp_http_session_test.go — update lines 191, 382.

Acceptance criteria

• Exactly one occurrence of the string "2025-03-26" remains in production code — the value of the mcpProtocolVersion constant.
• The initialize response body is produced by a single function called from both stdio and HTTP dispatchers.
• TestMCPProtocol_Initialize and TestHTTPDispatch_Initialize pass with no assertion changes (behaviour is byte-identical).
• Test files no longer hardcode "2025-03-26" in more than one place per file (constant reference is fine).
• go build ./... and go vet ./... are clean.
• go test -race ./... passes.

Verification

cd plugins/corezoid/mcp-server
go build ./...
go vet ./...
go test -race -coverprofile=coverage.out ./...
grep -rn '"2025-03-26"' .       # should show 1 hit (the constant definition)

Dependencies

• Blocked by: None.
• Blocks: feat: implement server/discover method on stdio and HTTP transports #114, feat: return UnsupportedProtocolVersionError (-32022) on unknown initialize protocolVersion #98
• Related: Epic: MCP 2026-07-28 legacy-compat shim #111 (epic)

Notes

• mcpServerVersion (line 52 of mcp_server.go) is the server version (the plugin's 2.3.5), not the protocol version. Do not merge the two — they change on different cadences.
• The MCP 2026-07-28 spec explicitly lists 2025-03-26 as a valid legacy version and expects legacy servers to keep returning it verbatim in initialize. Do not bump.
• Keep the helper's return type as map[string]interface{} to avoid churning JSON marshalling for callers; a typed struct is not required at this stage.