fix(navigation): honour --enable-write-tools for generate_deeplink

[MehmetSecgin]

What this changes

On a server run with --disable-write, --enable-write-tools=generate_deeplink now registers the writable generate_deeplink, so shorten=true returns a /goto/<uid> link instead of being silently ignored. Before, AddNavigationTools received the global write flag, so the per-tool override never reached it.

Related: #820

Short URLs are a POST /api/short-urls that needs no RBAC action, so they fit the scoped opt-in that --enable-write-tools exists for. No tool name, input or output shape changes. Without the override, --disable-write still registers the read-only variant.

How you tested it

Unit tests only, no live Grafana instance.

• TestProcessTools_EnableWriteToolsRestoresWritableDeeplink: with --disable-write and the override, generate_deeplink is the writable variant (readOnlyHint: false) and other write tools such as update_dashboard stay unregistered. It fails on main.
• TestProcessTools_DisableWriteRegistersReadOnlyDeeplink: with --disable-write alone, the read-only variant is registered.

Checklist

• make lint and make test-unit pass locally
• Commits are signed (required to merge, see CONTRIBUTING.md)
• CLA signed (required to merge)
• README tool table updated, with RBAC permissions and scopes, if this adds a tool (n/a, no new tool)
• Any write operation respects --disable-write
• Any datasource query execution respects --disable-query (n/a, no query execution)

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