Integration credential modals should link to where users get required keys

[enyst]

Problem

Integration/MCP install modals ask users for credentials, but they do not consistently point users to the place where those credentials are created.

Concrete example from the weekend testing feedback:

• The Slack install flow asks for Slack credentials such as a bot token / workspace ID.
• The user is not pointed at Slack's app creation page: https://api.slack.com/apps/
• The current Slack catalog assertion only verifies docsUrl === https://github.com/zencoderai/slack-mcp-server, which explains the MCP package but does not give the user a clear credential-acquisition path.

This is adjacent to #926, but not the same issue:

• MCP Server configuration form should allow setting secrets needed by the Automation Server #926 is about saving integration credentials into the secrets store so automations can use them.
• This issue is about discoverability / setup guidance inside the integration modal.

Expected behavior

When an integration requires user-provided credentials, the modal should provide a clear, contextual link to where the user can obtain those credentials.

For Slack, that should include either:

• a direct link to https://api.slack.com/apps/, and/or
• the upcoming Agent Canvas docs page for the prebuilt Slack channel monitor setup.

The modal copy should make it clear which value goes into which field and where to create/find it.

Acceptance criteria

• Slack install/setup UI links users to the Slack app creation/configuration page or an Agent Canvas docs page that contains that link.
• The link is visible before the user submits the modal.
• Other credential-requiring marketplace integrations have a way to provide equivalent credential-source docs.
• Tests cover at least the Slack link so the modal does not regress.

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This issue was created by an AI agent (OpenHands) on behalf of the user.

[DevinVinson]

Duplicate of #979 although this is a bit more filled out. Here was my comment in 979 about where we can pull the info from the extensions repo data:

The OpenHands Extensions repo is where we pull Integrations from (MCP servers). The data should already have the helperLink and it would be helpful if we showed it to users: https://github.com/OpenHands/extensions/blob/main/integrations/catalog/slack.json#L36

Closing out 979 in favor of this one.