refactor(tools): use docker/aijson for LLM-tolerant JSON unmarshal

[trungutt]

The validate-then-repair layer added in #2635 has zero coupling to docker-agent. The same four shape repairs apply to every place we parse JSON produced by an LLM — tool-call arguments today, structured outputs and JSON-mode responses next. Keeping it in pkg/tools/ ties a reusable utility to a single import site and makes it harder to apply at other entry points (MCP/A2A bridges, future consumers).

This PR moves the implementation to github.com/docker/aijson and replaces the local copy with a one-line aijson.Unmarshal call. The library's public API is a drop-in for encoding/json.Unmarshal:

var args Args
if err := aijson.Unmarshal(data, &args); err != nil {
    return err
}

NewHandler[T] collapses to a single call that keeps the tool_input_repaired slog event for telemetry. The four repair kinds, their ordering invariant, and the validate-then-repair design all move with the implementation and are unit-tested in the new repo.

Behavior is unchanged; valid input still pays nothing.

[docker-agent]

Assessment: 🟡 NEEDS ATTENTION

This PR correctly removes the local repair layer and delegates to github.com/docker/aijson. The compile-time surface is clean — no missing symbols, no orphaned callers. One medium observability concern and one stale comment were flagged.

[docker-agent]

[MEDIUM] tool_input_repaired slog field type changed — telemetry representation may silently differ

The repairs slog field now logs []aijson.Kind (an external type from github.com/docker/aijson) instead of the previous []repairKind (local typed strings). Two concerns:

1. String representation: aijson.Kind values may render differently in structured logs / alerting pipelines that pattern-match on the old repair-kind strings (e.g. "json_stringified_array"). If any dashboards or alerts key on those strings, they will silently stop matching.

2. Firing semantics: The callback is now driven entirely by aijson.Unmarshal's internals. If the library's notion of "a repair was applied" differs from the old validate-then-repair loop (e.g., it fires for normalizations the old code did not count, or skips one of the four repair kinds in some edge case), the event rate will diverge without any compiler or test signal.

The PR description asserts "Behavior is unchanged" but this is not testable from the diff alone — the four repair kinds and their ordering invariant now live in an external v0.1.0 package. Consider adding a test that asserts the exact aijson.Kind string values emitted for each of the four repair cases, or explicitly documenting the expected Kind strings in the slog event.

[docker-agent]

[LOW] Stale comment references wrong package name (json-tool-repair vs aijson)

The comment says unit tests for the four repair kinds live in github.com/docker/json-tool-repair, but the actual library added to go.mod is github.com/docker/aijson. A future contributor following this comment will not find the referenced repository.

Suggested fix:

// These are integration tests for NewHandler's repair plumbing. Unit tests
// for the four repair kinds live in github.com/docker/aijson.