feat: filter chats by diff URL via the q search parameter (#24970)

Adds a `diff_url:` term to the `q` search parameter on `GET
/api/experimental/chats` so callers can look up the chat associated with
a particular pull request, merge request, or any other URL persisted on
the chat's diff status.

```
q=diff_url:"https://github.com/coder/coder/pull/123"
```

Match is case-insensitive. When the URL lives on a delegated sub-agent's
diff status, the parent chat is returned so the relationship surfaces
from a single lookup.

<details>
<summary>Design notes</summary>

- **Forge-agnostic.** Reuses the existing `chat_diff_statuses.url`
column rather than introducing a `pr:` vocabulary, since the SDK already
documents the URL as "may point to a pull request or a branch page
depending on whether a PR has been opened." Works for GitHub PRs, GitLab
MRs, branch pages, etc.
- **Composes with `archived:`.** The two terms can be combined:
`q=archived:true diff_url:"..."`.
- **Case handling.** The parser used to lowercase the entire `q` string
up front, which would mangle URL path segments. Switched to lowercasing
only the field key inside `searchTerms` (already happens there) and
keeping the value as the caller typed it. The SQL comparison lowercases
on both sides.
- **Validation.** `diff_url` must be a syntactically valid HTTP(S) URL
with a non-empty host. No forge-specific validation.
- **Index.** Adds `idx_chat_diff_statuses_url_lower` on `LOWER(url)` so
the lookup is cheap even on large datasets.
- **Sub-agent fan-in.** `EXISTS` clause matches when the URL lives on
the chat itself or any chat with `root_chat_id` equal to the chat's id,
so a delegated sub-agent's PR pulls in its parent.
- **Deferred.** Sentinels like `pr:any` / `pr:none` and a forge-agnostic
state filter (`diff_state:open|merged|closed`) were intentionally left
out of this change. They couple cleanly to a second forge or a clearer
product call, and shipping them now would lock in vocabulary we may want
to revisit.

</details>

## Tests

- `coderd/searchquery`: parser tests for valid URLs, case handling (key
insensitive, value preserved), composition with `archived:`, and
validation errors (non-HTTP scheme, missing host, malformed URL).
- `coderd/exp_chats_test.go`: end-to-end coverage hitting `ListChats`.
Verifies a root chat matches its own URL, a parent chat surfaces when
only a sub-agent has the URL, lookups are case-insensitive, non-matching
URLs return empty, and invalid URLs return `400`.

---

_This PR was authored by a Coder Agent on behalf of @kylecarbs._

coderd/database/queries/chats.sql

@@ -377,6 +377,22 @@ WHERE
         WHEN sqlc.narg('label_filter')::jsonb IS NOT NULL THEN chats.labels @> sqlc.narg('label_filter')::jsonb
         ELSE true
     END
+    -- Match chats whose linked diff URL (e.g. a pull request URL)
+    -- equals the given value, case-insensitively. The URL may live on
+    -- a delegated sub-agent's diff status, so we surface the root chat
+    -- when any descendant matches.
+    AND CASE
+        WHEN sqlc.narg('diff_url')::text IS NOT NULL THEN EXISTS (
+            SELECT 1
+            FROM chat_diff_statuses cds
+            JOIN chats c2 ON c2.id = cds.chat_id
+            WHERE cds.url IS NOT NULL
+              AND cds.url <> ''
+              AND LOWER(cds.url) = LOWER(sqlc.narg('diff_url')::text)
+              AND (c2.id = chats.id OR c2.root_chat_id = chats.id)
+        )
+        ELSE true
+    END
     -- Paginate over root chats only. Children are fetched
     -- separately via GetChildChatsByParentIDs and embedded under
     -- each parent. Other callers that need the full set should