Docs: replace non-Azure AsExisting examples with AddConnectionString

[IEvangelist]

Summary

This PR fixes the docs guidance behind #1074 by removing non-Azure AsExisting(...) examples that don't match available APIs.

The updated guidance now models existing external resources with AddConnectionString(...) / addConnectionString(...) instead.

What changed

• Replaced non-Azure existing-instance snippets that used AsExisting(connectionStringParameter: ...).
• Added C# and TypeScript examples using AddConnectionString / addConnectionString.
• Clarified that these values are resolved from AppHost configuration keys such as ConnectionStrings:<name> / ConnectionStrings__<name>.
• Clarified that this connection-string pattern does not inject typed resource-specific connection-property variables.

Files updated

• src/frontend/src/content/docs/integrations/caching/redis/redis-host.mdx
• src/frontend/src/content/docs/integrations/caching/garnet/garnet-host.mdx
• src/frontend/src/content/docs/integrations/caching/valkey/valkey-host.mdx
• src/frontend/src/content/docs/integrations/messaging/rabbitmq/rabbitmq-host.mdx
• src/frontend/src/content/docs/integrations/databases/elasticsearch/elasticsearch-host.mdx
• src/frontend/src/content/docs/get-started/glossary.mdx

Fixes #1074

[Copilot]

Pull request overview

Updates Aspire integration docs to stop recommending non-existent/non-exposed AsExisting(...) patterns for non-Azure resources, replacing them with connection-string based modeling to address issue #1074.

Changes:

• Replace C# AddX(...).AsExisting(...) examples with AddConnectionString(...) across several integration Host docs.
• Replace TypeScript explanatory text with concrete addConnectionString(...) + withReference(...) examples.
• Update the glossary “Existing resource pattern” snippet to use AddConnectionString(...).

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
src/frontend/src/content/docs/integrations/caching/redis/redis-host.mdx	Switch existing-Redis example to connection-string resource; adds TS code sample.
src/frontend/src/content/docs/integrations/caching/garnet/garnet-host.mdx	Switch existing-Garnet example to connection-string resource; adds TS code sample.
src/frontend/src/content/docs/integrations/caching/valkey/valkey-host.mdx	Switch existing-Valkey example to connection-string resource; adds TS code sample.
src/frontend/src/content/docs/integrations/messaging/rabbitmq/rabbitmq-host.mdx	Switch existing-RabbitMQ example to connection-string resource; adds TS code sample.
src/frontend/src/content/docs/integrations/databases/elasticsearch/elasticsearch-host.mdx	Switch existing-Elasticsearch example to connection-string resource; adds TS code sample.
src/frontend/src/content/docs/get-started/glossary.mdx	Update “Existing resource pattern” snippet to use AddConnectionString(...).

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[IEvangelist]

Addressed the Copilot review feedback in b51f231:

• Added clarification in each updated integration page that AddConnectionString / addConnectionString resolves values from ConnectionStrings:<name> (or ConnectionStrings__<name>) in AppHost configuration.
• Clarified that this pattern injects a connection string value rather than typed resource-specific connection-property variables.
• Added the same configuration-source note to the glossary AddConnectionString("db") example.

[mitchdenny]

I'm wondering whether it might be a more meaningful story if we show using AddGarnet locally and switching to AddConnectionString for deployment? The thing we need to figure out there is the differences in how the strings are handled because we won't deconstruct a connection string into REDIS_HOST and the like.

[IEvangelist]

Follow-up update in bfca595:

I strengthened the wording in each affected section to explicitly call out format behavior:

• ConnectionStrings__{name} / ConnectionStrings:{name} is a single pass-through connection string value.
• It is not deconstructed into resource connection-property variables.
• Each page now includes concrete examples of the resource-specific variables that are not emitted in this pattern.

[IEvangelist]

Expanded scope in 02ec0e7 to the corresponding *-connect pages (Redis, Garnet, Valkey, RabbitMQ, Elasticsearch).

Each now explicitly documents the distinction:

• connection-property env vars are available when the AppHost models a typed resource.
• AddConnectionString / addConnectionString provides a single ConnectionStrings value (ConnectionStrings:{name} / ConnectionStrings__{name}), and it is not deconstructed into per-property variables.