GO-5364 export markdown in memory

[requilence]

Internal export API to export a single object into markdown byte slice. To be used in API

[github-actions]

	🟢 SMOKE-TEST PASSED
Tests	✔️ 8 tests run
Summary	🟢 8 passed; 🟡 0 skipped
Duration	🕐 13 minutes, 1 seconds
Testomat.io Report	📊 Run #87155bbf
Job	🗂️ Smoke Tests / smoke-test
Operating System	🖥️ Linux X64

✅ Passed Tests (8)

• Revoking an invite link (3 minutes, 4 seconds)
• User cancels their join request (1 minute, 23 seconds)
• User deletes the space and rejoins later (1 minute, 36 seconds)
• Owner changes the rights of a user from Viewer to Editor (1 minute, 48 seconds)
• Owner approves a join request with Editor permissions (1 minute, 32 seconds)
• Owner declines a join request (1 minute, 8 seconds)
• Owner removes a participant from the space (1 minute, 23 seconds)
• Sync on staging nodes (1 minute, 4 seconds)

[Copilot]

Pull Request Overview

This PR implements an internal export API to export a single object into a Markdown byte slice for the API. Key changes include:

• Adding new export entries to the documentation and proto definitions.
• Creating a new middleware handler (ObjectExport) and its supporting ExportSingleInMemory method.
• Introducing an InMemoryWriter and deepLinkNamer to support in-memory and deep-linked exports.

Reviewed Changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
docs/proto.md	Added new export-related entries and hyperlinks for RPC calls.
pkg/lib/mill/image_resize.go	Introduced IsImageExt to support image extension checks.
core/export.go	Added ObjectExport middleware and ExportSingleInMemory for in-memory exports.
pb/protos/commands.proto	Added new Export Request/Response messages and updated comments in the proto.
core/block/export/export.go	Implemented exportObject for synchronous single-object export using InMemoryWriter.
core/block/import/markdown/anymark/anyblocks.go	Replaced hardcoded image extension list with mill.IsImageExt.
pb/protos/service/service.proto	Added ObjectExport RPC definition.
core/block/export/writer.go	Updated writer interface and added InMemoryWriter and deepLinkNamer for link exports.

Comments suppressed due to low confidence (2)

pb/protos/commands.proto:2879

• [nitpick] Consider updating the comment to 'ID of document for export' to match the singular field name 'objectId' and improve clarity.

                // ids of documents for export, when empty - will export all available docs

core/block/export/writer.go:203

• Ensure that the 'ext' parameter is consistently formatted (with or without a leading period) when used in condition checks. For example, if ext is not normalized prior to comparison, deepLinkNamer.Get may produce unexpected results.

if ext == ".md" {

[Copilot]

The loop in exportObject returns on the first iteration; if multiple file entries are ever present, only the first will be processed. Consider asserting that only one file entry exists or explicitly handling multiple entries.

Suggested change

	for _, v := range inMemoryWriter.data {
	if e.format == model.Export_Protobuf {
	return base64.StdEncoding.EncodeToString(v), nil
	}
	return string(v), nil
	}
	return "", fmt.Errorf("failed to find data in writer")
	var result string
	for _, v := range inMemoryWriter.data {
	if e.format == model.Export_Protobuf {
	result += base64.StdEncoding.EncodeToString(v)
	} else {
	result += string(v)
	}
	}
	if result == "" {
	return "", fmt.Errorf("failed to find data in writer")
	}
	return result, nil