feat(markdown): support Telegram ||spoiler|| syntax

[ernado]

Summary

The message/markdown parser is CommonMark (goldmark) and had no spoiler
construct, so Telegram's MarkdownV2 ||text|| was emitted as literal text
instead of a spoiler entity.

This adds a goldmark inline parser for the ||...|| delimiter (exactly two
pipes), mapping it onto the messageEntitySpoiler entity. It's modeled on the
existing strikethrough delimiter parser.

Changes

• spoiler.go — NewSpoilerParser() inline parser + spoiler AST node /
  delimiter processor (trigger |, requires a run of exactly 2).
• parser.go — register the parser (priority 500) and render the node via
  entity.Spoiler().
• markdown.go — document ||text|| spoiler in the package overview.
• parser_test.go — a Spoiler test group: basic, mid-text, nested
  (||spoiler with *italic*||), single-pipe-stays-literal, and escaped pipes.

Behavior

||spoiler||              -> spoiler entity over "spoiler"
before ||hidden|| after  -> spoiler over "hidden"
a | b                    -> plain text (single pipe is not a delimiter)
\|\|x\|\|                -> literal "||x||"

All telegram/message/... tests pass; go vet and gofmt clean.

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 85.18519% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 71.19%. Comparing base (5bd59ff) to head (de06034).
⚠️ Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
telegram/message/markdown/spoiler.go	83.33%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1775      +/-   ##
==========================================
+ Coverage   71.10%   71.19%   +0.08%     
==========================================
  Files         501      503       +2     
  Lines       23501    23574      +73     
==========================================
+ Hits        16711    16783      +72     
+ Misses       5569     5564       -5     
- Partials     1221     1227       +6     

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