refactor: improve robustness, eliminate duplication, and align with Go idioms

[Copilot]

The codebase had several recurring issues: magic numbers, copy-pasted HTTP client setup across three model constructors, a non-standards-conforming error helper that skipped error-chain unwrapping, operator-precedence ambiguity in prefix matching, and silently discarded errors in the agent's hot path.

Correctness

• httpx: Replaced home-grown errorAs() (no chain unwrapping, used deprecated net.Error.Temporary()) with stdlib errors.As
• session: len(k) >= len(prefix) && k[:len(prefix)] == prefix → strings.HasPrefix(k, prefix) (operator-precedence bug, applied to both MemorySession and JSONSession)

Duplication → shared abstraction

All three model constructors repeated identical HTTP client initialization. Extracted into model/http.go:

// Before (repeated verbatim in openai.go, dashscope.go, anthropic.go)
if client == nil {
    client = &http.Client{Timeout: 60 * time.Second}
}

// After
const defaultModelTimeout = 60 * time.Second

func defaultHTTPClient(c *http.Client) *http.Client {
    if c != nil {
        return c
    }
    return &http.Client{Timeout: defaultModelTimeout}
}

Default base URLs and API version strings promoted to named constants (defaultOpenAIBaseURL, defaultDashScopeBaseURL, defaultAnthropicBaseURL, defaultAnthropicVersion, defaultAnthropicMaxOutputTokens).

Silent failures → observable warnings

react_agent.go was discarding memory-save and print errors with _ =. These are now logged via logrus.Warn so failures surface in production logs.

Miscellaneous

• message/msg.go: TimestampFormat constant replaces duplicated layout string; NewMsg type guard simplified to a switch; strings.Join replaces manual loop in GetTextContent
• react_agent.go: magic 4 and 3 → defaultMaxIters / defaultTopK; retrieveKnowledge uses strings.Builder instead of += concatenation
• httpx.go: defaultMaxAttempts / defaultBaseBackoff constants replace inline literals

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