Add wire Debug Level for LLM Request/Response Logging

[ksylvan]

Add wire Debug Level for LLM Request/Response Logging

Closes #2018

Summary

This PR introduces a new debug level, wire (level 4), that logs the full HTTP-style request/response exchanges between Fabric and LLM backends. This is the most verbose logging tier and sits above the existing trace level. The change spans the logging package, the core chatter logic, the CLI flag definition, and all supported i18n locale files.

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Files Changed

File	Change Type	Description
internal/log/log.go	Modified	Added Wire constant, updated LevelFromInt, added GetLevel() accessor
internal/log/log_test.go	Added	Unit tests for LevelFromInt covering all levels including the new Wire level
internal/core/chatter.go	Modified	Injected Wire-level debug logging at three key points in the LLM communication lifecycle
internal/cli/flags.go	Modified	Updated --debug flag description to document level 4
internal/i18n/locales/*.json	Modified	Updated set_debug_level description string across all 10 locale files

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Code Changes

internal/log/log.go — New Wire Level and GetLevel() Accessor

// Wire logs full request/response exchanges with model backends.
Wire

The LevelFromInt function was updated to handle the new level precisely:

case i == 3:
    return Trace
case i >= 4:
    return Wire

Previously i >= 3 returned Trace, meaning there was no way to distinguish between 3 and any higher integer. This is now fixed with an exact match for 3 and a catch-all >= 4 for Wire.

A new thread-safe GetLevel() function was added:

func GetLevel() Level {
    mu.RLock()
    defer mu.RUnlock()
    return level
}

This avoids callers needing to reach into unexported state and ensures proper read-lock semantics.

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internal/core/chatter.go — Wire-Level Logging at LLM Boundaries

Three logging points were added to capture the full conversation at the wire level:

1. Outbound messages before sending to LLM:

if debuglog.GetLevel() >= debuglog.Wire {
    debuglog.Debug(debuglog.Wire, "FABRIC->LLM request messages (%d)\n", len(vendorMessages))
    for i, msg := range vendorMessages {
        debuglog.Debug(debuglog.Wire, "FABRIC->LLM [%d] role=%s content=%q\n", i, msg.Role, msg.Content)
        if len(msg.MultiContent) > 0 {
            debuglog.Debug(debuglog.Wire, "FABRIC->LLM [%d] parts=%d\n", i, len(msg.MultiContent))
        }
    }
}

2. Each streaming response update:

if debuglog.GetLevel() >= debuglog.Wire {
    debuglog.Debug(debuglog.Wire, "LLM->FABRIC stream update type=%s content=%q\n", update.Type, update.Content)
    if update.Usage != nil {
        debuglog.Debug(debuglog.Wire, "LLM->FABRIC stream usage input=%d output=%d total=%d\n", ...)
    }
}

3. Non-streaming (synchronous) response:

if debuglog.GetLevel() >= debuglog.Wire {
    debuglog.Debug(debuglog.Wire, "LLM->FABRIC response content=%q\n", message)
}

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internal/log/log_test.go — New Test File

A table-driven unit test covers all levels including edge cases:

{in: -1, want: Off},
{in: 0, want: Off},
{in: 4, want: Wire},
{in: 9, want: Wire},  // any value >= 4 maps to Wire

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Reason for Changes

Debugging LLM integration issues (e.g., malformed prompts, unexpected model responses, token usage discrepancies) previously required adding temporary ad-hoc logging or attaching a network proxy. The wire level formalises this as a first-class debug capability, making it straightforward to capture the exact data being exchanged with model backends without modifying application code.

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Impact of Changes

• No functional change for users running at debug levels 0–3. The >= 4 guard ensures zero overhead unless wire logging is explicitly enabled.
• The LevelFromInt fix for the i >= 3 edge case is a subtle but correct behaviour change: previously, passing --debug 4 (or any value above 3) would have silently returned Trace. Now it correctly returns Wire. Any existing automation or scripts that passed --debug 4 expecting Trace behaviour will now receive Wire verbosity instead — this is the intended and more correct behaviour.
• The GetLevel() accessor is additive and non-breaking.
• Log output at wire level may contain sensitive data (full prompt content, model responses), which is appropriate for a local debug tool but worth noting for environments where logs are forwarded to external systems.

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Test Plan

• Run go test ./internal/log/... — the new TestLevelFromInt test should pass covering all level mappings.
• Run Fabric with --debug 4 and confirm FABRIC->LLM and LLM->FABRIC lines appear on stderr.
• Run Fabric with --debug 3 and confirm no wire-level output is produced.
• Test both streaming and non-streaming model paths (streaming via UpdateChan, synchronous via direct Send).
• Verify --help output and locale-specific help strings reflect 4=wire.

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Additional Notes

• The wire term is borrowed from network-layer logging conventions (e.g., "wire format"), making the intent self-documenting for engineers familiar with distributed systems debugging.
• The 4=wire label in the i18n locale files is intentionally left untranslated (wire) across all languages, consistent with how technical debug terms are typically handled in developer tooling.
• Multi-content (multimodal) messages are partially logged — only the part count is emitted, not the raw binary content of each part. This is a reasonable tradeoff to avoid flooding logs with base64-encoded image data.