fix(agent-sdk): strip trailing colon from URL scheme in createRemoteAttachment

[ygd58]

Fixes #4034

createRemoteAttachment() stored url.protocol directly as the remote attachment scheme. URL.protocol includes the trailing colon (e.g. "https:"), but the rest of libxmtp's remote attachment tests and content type examples consistently use the scheme without it (e.g. "https") - bindings/node/test/RemoteAttachmentEncryption.test.ts, bindings/wasm/test/RemoteAttachmentEncryption.test.ts, bindings/wasm/test/EnrichedMessage.test.ts, and crates/xmtp_content_types/src/remote_attachment.rs.

Fix: strip the trailing colon with url.protocol.replace(/:$/, ""), matching the suggested fix in the issue.

Tests: added a regression test asserting the scheme for both https and http URLs. Verified it fails against the pre-fix code (expected 'https:' to be 'https') and passes against the fix, via a full native build (cargo build for bindings/node, then yarn build for node-sdk, since agent-sdk's test suite imports encryptAttachment from @xmtp/node-sdk, which requires the compiled native binding).

Verified locally: vitest run src/util/AttachmentUtil.test.ts - 3/3 passing, typecheck, eslint, and prettier clean on both changed files.

Note

Strip trailing colon from URL scheme in createRemoteAttachment

URL.protocol returns a trailing colon (e.g. https:), which was being stored as the attachment scheme. The fix removes that colon so schemes like https and http are stored cleanly. Tests now assert scheme normalization for both HTTPS and HTTP URLs.

^{Macroscope summarized fd9db85.}