feat(proxy): add sentinel errors and unified error handling (fixes #89)

[UmarFarooq-MP]

What this PR does

This PR implements centralized error handling for the proxy layer in lokxy.

Key Changes

• Added a new pkg/proxy/errors package with sentinel errors (e.g. ErrNoUpstream, ErrReadBodyFailed, ErrTailRequiresWebSocket).
• Introduced WriteJSONError utility so that all handlers return consistent JSON error responses instead of raw text/plain.
• Updated all proxy handlers (query, labels, series, stats, tail) to use the centralized error definitions.
• Ensured successful paths still forward valid Loki JSON responses without modification.
• Added shell command examples in README.md so users can easily test both success and failure cases for every endpoint.

Why this matters

• Users (and Grafana) now receive predictable JSON error payloads instead of raw error strings.
• Easier debugging: each error is clearly named, and log messages match user-facing error responses.
• Cleaner code: handlers no longer duplicate ad-hoc error handling logic.

Testing the Proxy Endpoints

To verify that lokxy is correctly proxying requests to Loki, you can run a few simple curl and websocat commands.
These examples assume:

• lokxy is running on http://localhost:3100
• Loki is running on http://localhost:9090
• You already have logs (e.g., from the included log-generator container)

---

Query Range (Success)

START=$(( ($(date +%s) - 300) * 1000000000 ))

curl -sG \
  --data-urlencode 'query={container="lokxy-log-generator-1"}' \
  --data-urlencode "limit=5" \
  --data-urlencode "start=$START" \
  http://localhost:3100/loki/api/v1/query_range | jq .

curl -sG \
  --data-urlencode 'query={app=~".*"}' \
  --data-urlencode "limit=5" \
  --data-urlencode "start=$START" \
  http://localhost:3100/loki/api/v1/query_range

curl -s http://localhost:3100/loki/api/v1/labels | jq .

curl -sG \
  --data-urlencode 'match[]={container="lokxy-log-generator-1"}' \
  http://localhost:3100/loki/api/v1/series | jq .

websocat "ws://localhost:3100/loki/api/v1/tail?query={container=\"lokxy-log-generator-1\"}&limit=5"

---

This way your test.sh remains local for you, and contributors can still see exactly how to test without needing a script.

Closes #89

[UmarFarooq-MP]

is it wise to update the tool chain to latest on 1.24 but CI needs to update for that otherwise CI will keep on failing. @heliapb

[paulojmdias]

@UmarFarooq-MP thanks for your PR! Can we just focus on returning back the errors returned by Loki? Currently it seems a big refactor

[UmarFarooq-MP]

I think you are right let me revert them. I added a response pattern with sentinel errors. @paulojmdias

[UmarFarooq-MP]

The PR is updated @paulojmdias @heliapb

[paulojmdias]

@UmarFarooq-MP can we keep all the existing comments to make this PR easier to review?

[UmarFarooq-MP]

sure @paulojmdias , was caught up in thing will try to do it in this week.

[paulojmdias]

@UmarFarooq-MP any chance to came back on this ? 🙏