fix(suggestion): avoid echoing the current query

[hjxccc]

Description

Prevent follow-up suggestions from repeating the user's current question after normalization.

The knowledge path can mechanically turn an entity or summary Wiki page titled X into 介绍一下X. When that is also the current query, relevance ranking promotes the echo because its context starts with CurrentQuery, and the hybrid merge reserves a knowledge slot for it. The model path can also ignore the prompt-level instruction not to repeat prior questions.

This change:

• filters normalized exact matches against CurrentQuery on both model and knowledge paths;
• filters knowledge candidates inside the selection loop so later candidates can backfill the configured count;
• deliberately avoids fuzzy similarity thresholds so valid deeper questions about the same entity remain available;
• adds coverage for punctuation, whitespace, case, empty-query behavior, and hybrid slot preservation.

Type of Change

• 🐛 Bug fix
• ✨ New feature
• 💥 Breaking change
• 📚 Documentation update
• 🎨 Refactor
• ⚡ Performance improvement
• 🧪 Test
• 🔧 Configuration / Build / CI

Related Issue

Fixes #2519

Testing

• gofmt -l internal/application/service/message_suggestion.go internal/application/service/message_suggestion_test.go — no output
• go test ./internal/application/service/ -run 'Suggestion|FollowUp|MergeHybrid' -count=1 — passed with Go 1.26
• go vet ./internal/application/service/ — passed with Go 1.26
• git diff --check upstream/main...HEAD — passed

The focused backend package was validated. Full-repository make test was not run because this change is isolated to follow-up suggestion generation and its unit tests.

Checklist

• git diff --check origin/main...HEAD passes
• Changed source files are formatted
• Targeted tests for the changed packages/components pass
• Diff-scoped lint passes where applicable (validated with go vet for the changed package)
• Full-repository checks were run, or any unrelated/environment-dependent failures are documented above
• Self-reviewed the code
• Added/updated tests covering the change
• Updated related documentation (not applicable; no user-facing contract or configuration change)
• Breaking changes are clearly called out (none)

Screenshots / Recordings

Not applicable; this is a backend-only fix.

[hjxccc]

友好 ping 一下 🙏 这个 PR 开了 11 天了，CI 全绿、无冲突。

修的是 #2519：知识路径会把标题为 X 的 Wiki 页机械地变成 介绍一下X，当它正好等于当前问题时，相关性排序反而会把这条回显推到前面。改动只过滤「归一化后与 CurrentQuery 完全相同」的候选，不做模糊相似度，所以同一实体的深入追问不受影响。

需要我 rebase 到最新 main、或者把改动拆得更小，随时说，我会立刻跟进。

[iuyo5678]

抱歉拖了这么久才回复，整个改动没有问题，不过我看PR 信息你是从实际数据库中导出的，这些信息会涉及你的一些个人身份信息，建议测试用例简单做个修改，换成一些通用的日常表达，即可merge了。🙏

Sorry for taking so long to reply，there are no problem with the changes overall，but I see from the PR information that you exported this from an actual knowledge database，and this information may involve some of your personal identifying information，I suggest making a simple change to the test cases，replacing it with some generic everyday expressions，then it can be merged。🙏