Limit filtered deployment pages to request size

[rootp1]

What this PR does:

• limits ListDeployments responses to the requested filtered page size when label filters are applied
• returns a resume cursor that can continue from the middle of a datastore page without skipping matching deployments
• adds regression tests for mid-page resume and exact datastore page-boundary behavior

Why we need it:

• the current handler can return more than one logical page of filtered deployments in a single response
• the returned datastore cursor alone is not a stable resume point when the filtered page ends in the middle of a fetched datastore page

Which issue(s) this PR fixes:

Fixes #7074

Does this PR introduce a user-facing change?:

• How are users affected by this change: filtered deployment pagination now respects the requested page size and follow-up requests continue without duplicates or gaps
• Is this breaking change: no
• How to migrate (if breaking change): not applicable

Summary

• enforce the requested logical page size after label filtering in WebAPI.ListDeployments
• encode an internal resume cursor only when a filtered page ends mid-datastore-page
• cover the new cursor behavior with focused grpcapi regression tests

Linked Issue

Fixes #7074

What Changed

• added a private cursor format for ListDeployments filtered pagination resumes
• stopped appending matches once the filtered page reaches PageSize
• added tests for stable filtered pagination across multiple datastore pages

Verification

• go test ./pkg/app/server/grpcapi -run 'TestListDeploymentsWithLabels' — passed
• cd pkg/app/server/grpcapi && /Users/rootp1/go/bin/golangci-lint run -v --config /Users/rootp1/Documents/repos/n/new/pipecd-issue2/.golangci.yml ./... — passed
• make update/web-deps — passed
• make check — failed: repository-pinned golangci-lint image is built with Go 1.25.0 while this repo targets Go 1.26.2, so lint aborts before analysis
• /Users/rootp1/go/bin/golangci-lint run -v --config .golangci.yml across repo modules — failed: unrelated existing goheader issue in web/node_modules/flatted/golang/pkg/flatted/flatted.go after installing web dependencies
• make check/gen/code — passed
• make check/dco — failed: existing upstream commit 25ae69937e55e8330bff96c34346718f3e5e7148 is missing a Signed-off-by line
• make test — not completed: root go test -failfast -race ./pkg/... ./cmd/... remained running without further output for several minutes on this machine and was stopped after targeted checks had already passed

Scope

• only pkg/app/server/grpcapi/web_api.go and pkg/app/server/grpcapi/web_api_test.go were changed for this fix