refactor(client): introduce StripLeadingSlash type

[yusukebe]

The author should do the following, if applicable

• Add tests
• Run tests
• bun run format:fix && bun run lint:fix to format the code
• Add TSDoc/JSDoc to document the code

[github-actions]

HTTP Performance Benchmark

Framework	Runtime	Average	Ping	Query	Body
hono (origin/main)	bun	51,239.27	67,271.79	47,403.07	39,042.95
hono (current)	bun	50,271.88	65,741.68	45,857.28	39,216.68
Change		-1.89%	-2.27%	-3.26%	+0.44%

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.56%. Comparing base (41ee1bb) to head (0151bee).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4594   +/-   ##
=======================================
  Coverage   91.56%   91.56%           
=======================================
  Files         173      173           
  Lines       11256    11256           
  Branches     3267     3267           
=======================================
  Hits        10307    10307           
  Misses        948      948           
  Partials        1        1           

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[github-actions]

Bundle size check

	main (41ee1bb)	#4594 (c97289e)	+/-
Bundle Size (B)	18,377B	18,377B	0B
Bundle Size (KB)	17.95K	17.95K	0K

Compiler Diagnostics (tsc)

	main (41ee1bb)	#4594 (c97289e)	+/-
Files	303	303	0
Lines	141,136	141,140	4
Identifiers	128,893	128,896	3
Symbols	269,855	269,657	-198
Types	206,536	205,935	-601
Instantiations	666,325	665,377	-948
Memory used	352,492K	353,317K	825K
I/O read	0.03s	0.03s	0s
I/O write	0s	0s	0s
Parse time	0.64s	0.65s	0.01s
Bind time	0.27s	0.27s	0s
Check time	2.37s	2.47s	0.1s
Emit time	0s	0s	0s
Total time	3.28s	3.39s	0.11s

Compiler Diagnostics (typescript-go)

	main (41ee1bb)	#4594 (c97289e)	+/-
Files	303	303	0
Lines	141,138	141,142	4
Identifiers	128,856	128,859	3
Symbols	392,321	390,823	-1,498
Types	378,666	378,069	-597
Instantiations	1,077,904	1,076,994	-910
Memory used	257,972K	257,695K	-277K
Memory allocs	2,719,803	2,718,193	-1,610
Config time	0.001s	0.001s	0s
Parse time	0.089s	0.12s	0.031s
Bind time	0.034s	0.022s	-0.012s
Check time	0.571s	0.711s	0.14s
Emit time	0s	0s	0s
Total time	0.735s	0.898s	0.163s

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Reported by octocov