fix(sse): merge purify_history compression notice into the leading system message

[ggdayup]

Summary

• purifyHistory() (Layer 3 of compressContext) spliced its [Context compressed: N earlier messages removed to fit context window] notice as a second system message at index system.length — i.e. after the leading system block, in the middle of the array.
• Gateways that accept a system role only at index 0 reject that with HTTP 400 System message must be at the beginning. Confirmed live against the built-in TokenRouter gateway (2026-08-22): 10 call logs show combo-routed requests failing exactly when this notice was present, and the per-provider circuit breaker then flapped the combo onto the next target, producing intermittent failures.
• Fix: merge the notice into the leading system/developer message (string content gets it prepended; content-parts arrays get a text part prepended). When the history has no leading system/developer message, prepend one at index 0 — accepted by every provider, and the same slot the old splice used when system[] was empty.
• No internal path emits a non-leading system message anymore, for any provider. Side benefits: one fewer message in compressed requests (stable count for downstream invariants), and the leading-system prefix stays contiguous for prompt caching.

Related Issues

• Related to fix(backend): cache-safe memory injection inserts 'system' at non-zero index → strict providers 400 #6135 (PROVIDERS_SYSTEM_MUST_BE_FIRST, same class of failure from memory injection) and fix(backend): hoist client-injected 'system' messages to index 0 for strict providers — #6135 only covered the memory half #7293 (strict-system hoist normalization).
• Companion PR adds tokenrouter to the strict list so the fix(backend): hoist client-injected 'system' messages to index 0 for strict providers — #6135 only covered the memory half #7293 hoist also normalizes any residual mid-array system messages from client histories.

Validation

• Change type: routing / other (SSE context-compression pipeline)
• Focused tests and category gates from the golden path
• npm run lint
• Reconciled with the current active release base (release/v3.8.50)
• Production-code changes include a new or updated automated test in this PR

Focused run (node --test, tsx loader):

tests/unit/context-manager-purify-system-first.test.ts   4/4 pass (new)
tests/unit/8594-compress-image-token-stringify.test.ts   pass
tests/unit/compression/responses-orphan-tool-call.test.ts pass
tests/unit/role-normalizer.test.ts                        pass
tests/unit/8560-responses-image-compaction.test.ts        pass

Tests Added Or Updated

• Added tests/unit/context-manager-purify-system-first.test.ts — forces Layer-3 purify_history and asserts: (1) the notice is merged INTO an existing leading system message (no second system role anywhere), (2) a single system message is prepended at index 0 when none exists, (3) a leading developer message receives the merge without gaining a second developer/system entry, (4) no compression ⇒ untouched history.

Coverage Notes

• Changes open-sse/: covered by the new test above plus the existing compression suites listed under Validation (#8594 image-estimate behavior, orphan-tool-call restoration, role-normalizer notice-text handling all unchanged).

Reviewer Notes

• The old splice position (result.splice(system.length, …)) is what made the notice land after every system/developer message but before user turns; the replacement keeps the notice as close to the front as possible while guaranteeing index-0-only system roles.
• Content-parts arrays on the leading system message are handled by unshifting a {type:"text"} part, preserving existing parts (e.g. cache_control markers) untouched.