feat: add BOBS write routing and stress streaming

[jameshawkes]

Summary

• update frontend to current BITS request-id/overload API and return retryable 529 responses on broker overload
• update worker BOBS delivery to use write_url returned by /create
• split BOBS create/body clients so /create gets fresh kube load-balancing while write/complete reuse HTTP/2 connections
• use h2c HTTP/2 prior knowledge for worker-to-BOBS body delivery
• return BOBS read_url unchanged, so public result URLs remain opaque /download-N/{key} links
• pin the integration-test BOBS dependency to the merged BOBS commit that includes write routing and opaque public URLs
• add request-driven test-worker stress behaviour with lazy chunked output
• update worker/integration tests for routed BOBS writes

Depends on merged BOBS support in ecmwf/bobs#2 and ecmwf/bobs#3, plus chart routing in ecmwf/polytope-chart#40.

Verification

• cargo fmt --all -- --check
• cargo test -p polytope-worker-common bobs -- --nocapture
• cargo test -p polytope-server-integration-tests -- --nocapture → 27 passed
• cargo test -p polytope-server-integration-tests bobs_delivery_pipeline -- --nocapture
• rust-analyzer diagnostics for tests/integration/src/lib.rs: none
• built and pushed dev images during live validation: test-worker:majh-dev, mars-worker:majh-dev, fdb-worker:majh-dev, polytope-fe-worker:majh-dev
• live tier-2 stress suite passed against bologna-dev
• live BOBS full-range benchmark passed against the earlier /api/v1 public URL shape: 256/256 full-range downloads, 4 GiB produced/read, 211 MiB/s