fix(logql): enforce fully-anchored label regex semantics to match Prometheus (#14433)

[hazel-shen]

What this PR does / why we need it:

Align LogQL label/stream regex matchers with Prometheus semantics: regex patterns are treated as fully anchored (implicitly ^…$). This removes unintended substring matches in label filters, restoring the documented behavior. Line filters (|~, !~) remain substring-based and are unchanged.

Which issue(s) this PR fixes:
Fixes #14433

Special notes for your reviewer:

• Behavior recap with test="1234":
  test=~"1234" → match; test=~"23.*" → no match; test=~".*23" → no match; test=~"12.*" → match; test=~".*" → match (no-op); test!~".*" → no match.
• Added table-driven tests covering: Regex label filter return wrong result #14433 regressions (23.*, .*23), .* fast path and !~".*", half-anchors (^12, 34$), and missing label semantics ("",".*", ^$).
• References:
  - Issue: Regex label filter return wrong result #14433
  - Docs (fully anchored label filters): https://grafana.com/docs/loki/latest/query/log_queries/

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• Title matches the required conventional commits format, see here
  • Note that Promtail is considered to be feature complete, and future development for logs collection will be in Grafana Alloy. As such, feat PRs are unlikely to be accepted unless a case can be made for the feature actually being a bug fix to existing behavior.
• Changes that require user attention or interaction to upgrade are documented in docs/sources/setup/upgrade/_index.md
• If the change is deprecating or removing a configuration option, update the deprecated-config.yaml and deleted-config.yaml files respectively in the tools/deprecated-config-checker directory. Example PR

[CLAassistant]

All committers have signed the CLA.

[hazel-shen]

Manual verification

I validated the fully-anchored label regex semantics locally. For reviewers, here's a minimal repro script (uses ns timestamps and an isolated run label):

#!/bin/bash
LOKI=${LOKI:-http://localhost:3100}
RUN=$(date +%s); NOW=$RUN

# Push 4 test logs
for i in 0 1 2 3; do
  curl -sX POST "$LOKI/loki/api/v1/push" -H 'Content-Type: application/json' \
    -d '{"streams":[{"stream":{"run":"'$RUN'"},"values":[["'$NOW$i'00000000","test"]]}]}'>/dev/null
done
sleep 2

START=$((NOW-60)); END=$((NOW+60))

# Test: substring "23.*" should NOT match "1234" (issue #14433)
curl -sG "$LOKI/loki/api/v1/query_range" \
  --data-urlencode 'query={run="'$RUN'"} | label_format test="1234" | test=~"23.*"' \
  --data-urlencode "start=$START" --data-urlencode "end=$END" \
  | jq -r '.data.result[0].values | length // 0'  # Expected: 0

# Test: prefix "12.*" should match "1234"
curl -sG "$LOKI/loki/api/v1/query_range" \
  --data-urlencode 'query={run="'$RUN'"} | label_format test="1234" | test=~"12.*"' \
  --data-urlencode "start=$START" --data-urlencode "end=$END" \
  | jq -r '.data.result[0].values | length // 0'  # Expected: 4

Results:

• With fix: 0 and 4 ✓
• Before fix (bug): 4 and 4 ✗

I didn't commit the script to keep the PR minimal; happy to share a gist if useful.