Memory v2 PR-2: HTTP plugin client + circuit breaker + capability negotiation

[HongmingWang-Rabbit]

Builds on merged PR-1 (#2729).

Tracking issue: #2728
Task: #276

Summary

• HTTP client implementing every endpoint in the OpenAPI v1 contract
• /v1/health probe + capability negotiation; MCP handlers (PR-5) gate features on SupportsCapability
• Circuit breaker: 3 consecutive 5xx/transport-error failures → ErrBreakerOpen for 60s
• Validation runs at the boundary (every method calls body.Validate() before the wire)

Why these breaker constants

• 3 failures to open: long enough to skip transient blips, short enough to react before all in-flight handlers stack on the 2s timeout
• 60s cooldown: long enough to back off a flapping plugin, short enough that recovery is felt within a single user session
• 4xx does NOT trip: those are client bugs, not plugin-health issues; resets failure count instead

Coverage

100.0% of statements in client.go.

Edge cases tested

• Boot success + capability list registered
• Boot when plugin unreachable (Capabilities() stays nil)
• Refresh updates capabilities mid-session
• All 6 public endpoints reject invalid inputs server-side BEFORE making the HTTP call
• Standard error envelope decoding (404, 403, 500)
• Non-standard error body (5xx with plain-text body) wrapped into typed *contract.Error
• Empty error body with arbitrary status (418) → stub error with "empty body" marker
• 204 NoContent on endpoint expecting a body → zero-value response, no decode error
• Garbage JSON in 200 response → wrapped decode error
• Context cancellation propagates to transport
• Marshal error path (channel in Propagation field)
• NewRequest error path (unbalanced bracket in BaseURL)
• 4xx never opens the breaker
• 5xx opens the breaker after 3 consecutive
• Cooldown elapsed → next call goes through
• Success after cooldown resets failure count
• Successful interleave resets failure count even before threshold
• Open breaker blocks all 6 endpoints with ErrBreakerOpen

Test plan

• go test ./internal/memory/client/ -cover reports 100.0%
• Real HTTP round-trip via httptest covers wire layer wiring
• Synthetic transport tests cover fast paths

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