docs: fix typos in Doxygen comments and error messages - #25

Summary

Fixes 7 verified typos across 3 files. Each was checked against current master content before editing.

Doxygen tag errors (documentation rendering bugs)

• include/easyhttpcpp/Request.h: :brief Get tag. -> @brief Get tag. (colon instead of @, so the brief never renders)
• include/easyhttpcpp/Request.h: @Get request header -> @brief Get request header (@Get is not a Doxygen command)
• include/easyhttpcpp/Request.h: default of http method is get method. -> The default http method is the get method. (missing articles)

Message-string typos - please review carefully, these change runtime strings

• src/executorservice/QueuedThreadPool.cpp: task is not allow NULL -> task must not be NULL
• src/executorservice/QueuedThreadPool.cpp: corePoolSize is not allow 0 -> corePoolSize must not be 0
• src/executorservice/QueuedThreadPool.cpp: BlockingQueue is not allow NULL -> BlockingQueue must not be NULL
• src/common/LruCacheByDataSizeStrategy.cpp: log format dataDize=%zu -> dataSize=%zu

The last four touch exception/log message text rather than comments. I grepped the repo (including tests/) for these strings and found no test assertions that depend on them, but flagging it so maintainers can double-check.

No functional/behavioural changes beyond the message wording above; no identifiers renamed.

Note: I deliberately left the project's existing can not / Can not phrasing untouched throughout, since it is used consistently across the codebase and in CONTRIBUTING.md, and appears to be house style rather than an error.