Add public isrecoverable(::Exception) for retry classification (#1245)

[quinnj]

Closes #1245.

Downstream packages (notably GitHub.jl, reported by @DilumAluthge / @ericphanson) used HTTP.jl 1.x's HTTP.RetryRequest.isrecoverable(::Exception) to decide whether a request-path exception is worth retrying. The 2.0 rewrite dropped that module, and the equivalent logic survived only in the private _retryable_request_error.

This exposes it as a documented, public HTTP.isrecoverable(::Exception)::Bool that delegates to the same classifier the built-in retry policy uses — same name as the 1.x API to ease migration. It recognizes the transient transport/protocol failures (connection resets/EOF, socket errors, parse errors, dial/TLS-handshake timeouts, and the unwrapped causes of OpError/TLSError), treats a hit request deadline as non-recoverable, and accepts either the raw exception or the RequestRetryError wrapper handed to a retry_if callback.

HTTP.isrecoverable(EOFError())                       # true
HTTP.isrecoverable(ArgumentError("nope"))            # false
HTTP.isrecoverable(HTTP.RequestRetryError(EOFError())) # true — unwraps the retry_if wrapper

Minimal surface: a thin public wrapper over the existing internal function (no logic change, internal call sites untouched), documented in the client API reference, with tests covering recoverable/non-recoverable classification, the wrapper, and parity with the internal classifier.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.31%. Comparing base (b4b0a5e) to head (fcfd2c0).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1310   +/-   ##
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  Coverage   87.31%   87.31%           
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  Files          29       29           
  Lines       11289    11291    +2     
=======================================
+ Hits         9857     9859    +2     
  Misses       1432     1432           

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