refactor(mcp-clients): dedupe HTTP/Websocket and SSE transport setup

[pedrofrxncx]

Follows issue #4468 (simplify the MCP-mesh machinery).

createOutboundClient in apps/api/src/mcp-clients/outbound/index.ts had three near-identical blocks for the HTTP, Websocket, and SSE connection types: validate connection_url, build request headers, merge any custom headers, construct the transport, then compose it with AuthTransport + MonitoringTransport. Only the transport constructor (StreamableHTTPClientTransport vs SSEClientTransport) differed. Extracted the shared steps into one buildHttpLikeTransport(connection, ctx, superUser, virtualMcpId, TransportCtor) helper, parameterized only by the constructor.

A maintainer gets a smaller, single home for this logic instead of three call sites that must all be kept in sync by hand whenever header-building or transport composition changes.

Behavior-preserving: the error message stays ${connection.connection_type} connection missing URL — for the SSE branch this was already the literal string "SSE connection missing URL" before this change, since connection.connection_type is "SSE" in that branch, so the template produces the identical text. Header building, transport composition order (AuthTransport then MonitoringTransport), and the final ctx.getOrCreateClient call are all unchanged — this is a pure code-motion, no new branches or altered control flow.

Net diff: -54/+51 lines in one file.

Reviewer check: cd apps/api && bunx tsc --noEmit (clean) — there's no existing unit test isolating createOutboundClient (it needs a live/mocked MCP transport, which is e2e territory per this repo's testing tiers), so this is verified by typecheck + a line-by-line diff of the extracted logic against the original three blocks.

Locally ran: bun run fmt, cd apps/api && bunx tsc --noEmit, bunx oxlint apps/api/src/mcp-clients/outbound/index.ts (0 errors/warnings). Full CI validates the rest.

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Summary by cubic

Dedupes outbound HTTP/Websocket and SSE transport setup in createOutboundClient by extracting a shared helper, buildHttpLikeTransport. This reduces duplication and centralizes header and composition logic without changing behavior.

• The helper validates the URL, builds request headers, merges connection_headers.headers, constructs the transport, and composes with AuthTransport then MonitoringTransport.
• It is parameterized only by the transport constructor: StreamableHTTPClientTransport for HTTP/Websocket and SSEClientTransport for SSE.
• Preserves error text ("${connection.connection_type} connection missing URL"), header-building semantics, composition order, and ctx.getOrCreateClient usage.
• Review focus: confirm the helper mirrors the removed blocks, especially header merge order, SSE error string, and passing connectionId and virtualMcpId to MonitoringTransport.

^{Written for commit 05d8c2d. Summary will update on new commits.}