Add MCP server with OAuth/API-key auth (#6570)

Adds an optional MCP server (proprietary module) that exposes Stirling's
PDF operations and AI capabilities to MCP clients. Off by default, zero
footprint when disabled.

### What
- New `/mcp` endpoint: streamable-HTTP + JSON-RPC 2.0; 8 tools
(describe_operation, pages/convert/misc/security category tools, AI,
upload, download).
- Runs real operations over an internal loopback; results returned
inline as base64 (small) or by fileId (large).

### Auth (two modes)
- OAuth2 resource server: RFC 9728 protected-resource metadata, RFC 8707
audience binding, JWKS, `mcp.tools.read/write` scopes; binds each token
to a provisioned Stirling account.
- API-key mode: reuses Stirling per-user `X-API-KEY` (no IdP needed).

### Security
- Per-user file ownership in FileStorage: async/queued writes scoped to
the submitting user; legacy/owner-less files stay readable.
- Admin allow/block list controls which operations are exposed.
- Python engine gated behind a shared secret (`X-Engine-Auth`).
- MCP filter chain is isolated and cannot weaken the main app's
security.
- Hardened: no upstream error-body leakage, log injection sanitized,
fileId path/sidecar enumeration blocked.

### Config / footprint
- Disabled by default (`mcp.enabled=false`); all beans
`@ConditionalOnProperty`.
---

## Checklist

### General

- [ ] I have read the [Contribution
Guidelines](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/CONTRIBUTING.md)
- [ ] I have read the [Stirling-PDF Developer
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/DeveloperGuide.md)
(if applicable)
- [ ] I have read the [How to add new languages to
Stirling-PDF](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md)
(if applicable)
- [ ] I have performed a self-review of my own code
- [ ] My changes generate no new warnings

### Documentation

- [ ] I have updated relevant docs on [Stirling-PDF's doc
repo](https://github.com/Stirling-Tools/Stirling-Tools.github.io/blob/main/docs/)
(if functionality has heavily changed)
- [ ] I have read the section [Add New Translation
Tags](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md#add-new-translation-tags)
(for new translation tags only)

### Translations (if applicable)

- [ ] I ran
[`scripts/counter_translation.py`](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/docs/counter_translation.md)

### UI Changes (if applicable)

- [ ] Screenshots or videos demonstrating the UI changes are attached
(e.g., as comments or direct attachments in the PR)

### Testing (if applicable)

- [ ] I have run `task check` to verify linters, typechecks, and tests
pass
- [ ] I have tested my changes locally. Refer to the [Testing
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/DeveloperGuide.md#7-testing)
for more details.

.taskfiles/e2e.yml

@@ -212,3 +212,55 @@ tasks:
     desc: "Stop the SAML keycloak test environment"
     cmds:
       - docker compose -f testing/compose/docker-compose-keycloak-saml.yml down -v
+
+  mcp:up:
+    desc: "Start the MCP keycloak test environment (Stirling as OAuth resource server)"
+    summary: |
+      Brings up Keycloak (OAuth authorization server) + Stirling configured as an
+      MCP resource server, then you can exercise /mcp with real Keycloak tokens.
+      Set LICENSE_KEY=<KEY> to skip the interactive license prompt:
+        task e2e:mcp:up LICENSE_KEY=abc123
+      Pass extra flags via -- :
+        task e2e:mcp:up -- --validate --nobuild
+    ignore_error: true
+    cmds:
+      - bash testing/compose/start-mcp-test.sh {{if .LICENSE_KEY}}--license-key "{{.LICENSE_KEY}}"{{end}} {{.CLI_ARGS}}
+
+  mcp:manual:
+    desc: "Start the MCP keycloak test env in manual mode (prints URLs + a live token for your client)"
+    summary: |
+      Brings the stack up and prints copy-paste URLs/commands plus a freshly minted
+      access token so you can drive your own MCP client (Inspector, curl, ...).
+        task e2e:mcp:manual LICENSE_KEY=<your-license-key>
+      Add --nobuild if the images are already built:
+        task e2e:mcp:manual LICENSE_KEY=<your-license-key> -- --nobuild
+    ignore_error: true
+    cmds:
+      - bash testing/compose/start-mcp-test.sh --manual {{if .LICENSE_KEY}}--license-key "{{.LICENSE_KEY}}"{{end}} {{.CLI_ARGS}}
+
+  mcp:apikey:
+    desc: "Start the MCP test env in API-KEY manual mode (no OAuth/IdP): mints a key + prints client settings"
+    summary: |
+      Brings Stirling up in apikey auth mode and prints copy-paste client settings with a freshly
+      minted X-API-KEY - ideal for clients whose OAuth layer can't reach localhost.
+        task e2e:mcp:apikey LICENSE_KEY=<your-license-key>
+      Add --nobuild if images are already built:
+        task e2e:mcp:apikey LICENSE_KEY=<your-license-key> -- --nobuild
+    ignore_error: true
+    cmds:
+      - bash testing/compose/start-mcp-test.sh --apikey {{if .LICENSE_KEY}}--license-key "{{.LICENSE_KEY}}"{{end}} {{.CLI_ARGS}}
+
+  mcp:validate:
+    desc: "Validate the running MCP keycloak test environment end-to-end (oauth mode + real MCP SDK client)"
+    cmds:
+      - bash testing/compose/validate-mcp-test.sh
+
+  mcp:validate-apikey:
+    desc: "Validate the MCP server in API-KEY auth mode (mints a key + real MCP SDK client), then restore oauth"
+    cmds:
+      - bash testing/compose/validate-mcp-apikey.sh
+
+  mcp:down:
+    desc: "Stop the MCP keycloak test environment"
+    cmds:
+      - docker compose -f testing/compose/docker-compose-keycloak-mcp.yml down -v