fix: comprehensive bug fixes, hardening, and test coverage

Bug fixes:
- Fix nil-pointer panic in sendMessage SOAP fault handler (used err.Error() on nil)
- Fix missing return after SOAP fault response (caused fall-through to 200 OK)
- Fix timeout=0 shadowing: removed package-level constant, use configured value
- Fix receiveMessage not checking SOAP faults from CIIMS

Codec hardening:
- Replace fragile regex parsing with encoding/xml.Decoder for namespace-agnostic XML
- XML-escape all user inputs (user, pass, event) — not just message body
- Remove magic-number-based string slicing (split, GetErrMsg)

Transport improvements:
- Extract Client type with connection pooling (reuse http.Client across requests)
- Add CIIMSResponse domain type with IsFault(), ErrorMessage(), Messages()
- Add HTTPStatusError and ResponseTooLargeError typed errors
- Replace deprecated ioutil.ReadAll with io.ReadAll
- Add response body size limit (64 MiB)

Security hardening:
- Add MaxBytesReader (1MB request body limit) on both endpoints
- Add URL allowlist via CIIMS_ALLOWED_SERVERS env var
- Add normalizeBaseURL with scheme/host/query validation
- Add count bounds validation (1-1000) on /receive
- Add server-level timeouts (ReadHeaderTimeout, ReadTimeout, IdleTimeout)

Configuration:
- Introduce Config struct to replace package-level globals
- Add loadConfig() with full validation and error propagation
- Add getIntEnv() with positive-value enforcement

Test coverage (75 tests, 35 new):
- Phase 1: 8 internal tests (nil receiver, error types, SOAP+HTTP500, malformed XML)
- Phase 2: 19 pure function tests (normalizeBaseURL, parseAllowedServers, getIntEnv)
- Phase 3: 16 handler tests (send/receive success, SOAP fault, network error, HTTP 500,
  body too large, invalid JSON, missing fields, URL allowlist)
- Phase 4: 8 config/router tests (loadConfig, newServer, newRouter)

Toolchain:
- Upgrade Go 1.13 → 1.22, gin 1.6.3 → 1.10.0, testify 1.5.1 → 1.10.0

Cleanup:
- Remove dead code (unused post() function, commented-out defaults)
- Replace println with structured logging
- Add .gitignore
- Rewrite README with API docs, env vars, security considerations

go.sum

@@ -34,9 +34,6 @@ github.com/klauspost/cpuid/v2 v2.2.4 h1:acbojRNwl3o09bUq+yDCtZFc1aiwaAAxtcn8YkZX
 github.com/klauspost/cpuid/v2 v2.2.4/go.mod h1:RVVoqg1df56z8g3pUjL/3lE5UfnlrJX8tyFgg4nqhuY=
 github.com/leodido/go-urn v1.2.4 h1:XlAE/cm/ms7TE/VMVoduSpNBoyc2dOxHs5MZSwAN63Q=
 github.com/leodido/go-urn v1.2.4/go.mod h1:7ZrI8mTSeBSHl/UaRyKQW1qZeMgak41ANeCNaVckg+4=
-github.com/mattn/go-colorable v0.1.13 h1:fFA4WZxdEF4tXPZVKMLwD8oUnCTTo08duU7wxecdEvA=
-github.com/mattn/go-colorable v0.1.13/go.mod h1:7S9/ev0klgBDR4GtXTXX8a3vIGJpMovkB8vQcUbaXHg=
-github.com/mattn/go-isatty v0.0.16/go.mod h1:kYGgaQfpe5nmfYZH+SKPsOc2e4SrIfOl2e/yFXSvRLM=
 github.com/mattn/go-isatty v0.0.20 h1:xfD0iDuEKnDkl03q4limB+vH+GxLEtL/jb4xVJSWWEY=
 github.com/mattn/go-isatty v0.0.20/go.mod h1:W+V8PltTTMOvKvAeJH7IuucS94S2C6jfK/D7dTCTo3Y=
 github.com/modern-go/concurrent v0.0.0-20180228061459-e0a39a4cb421/go.mod h1:6dJC0mAP4ikYIbvyc7fijjWJddQyLn8Ig3JB5CqoB9Q=
@@ -64,10 +61,6 @@ github.com/twitchyliquid64/golang-asm v0.15.1 h1:SU5vSMR7hnwNxj24w34ZyCi/FmDZTkS
 github.com/twitchyliquid64/golang-asm v0.15.1/go.mod h1:a1lVb/DtPvCB8fslRZhAngC2+aY1QWCk3Cedj/Gdt08=
 github.com/ugorji/go/codec v1.2.11 h1:BMaWp1Bb6fHwEtbplGBGJ498wD+LKlNSl25MjdZY4dU=
 github.com/ugorji/go/codec v1.2.11/go.mod h1:UNopzCgEMSXjBc6AOMqYvWC1ktqTAfzJZUZgYf6w6lg=
-github.com/valyala/bytebufferpool v1.0.0 h1:GqA5TC/0021Y/b9FG4Oi9Mr3q7XYx6KllzawFIhcdPw=
-github.com/valyala/bytebufferpool v1.0.0/go.mod h1:6bBcMArwyJ5K/AmCkWv1jt77kVWyCJ6HpOuEn7z0Csc=
-github.com/valyala/fasttemplate v1.2.2 h1:lxLXG0uE3Qnshl9QyaK6XJxMXlQZELvChBOCmQD0Loo=
-github.com/valyala/fasttemplate v1.2.2/go.mod h1:KHLXt3tVN2HBp8eijSv/kGJopbvo7S+qRAEEKiv+SiQ=
 golang.org/x/arch v0.0.0-20210923205945-b76863e36670/go.mod h1:5om86z9Hs0C8fWVUuoMHwpExlXzs5Tkyp9hOrfG7pp8=
 golang.org/x/arch v0.3.0 h1:02VY4/ZcO/gBOH6PUaoiptASxtXU10jazRCP865E97k=
 golang.org/x/arch v0.3.0/go.mod h1:5om86z9Hs0C8fWVUuoMHwpExlXzs5Tkyp9hOrfG7pp8=
@@ -76,7 +69,6 @@ golang.org/x/crypto v0.9.0/go.mod h1:yrmDGqONDYtNj3tH8X9dzUun2m2lzPa9ngI6/RUPGR0
 golang.org/x/net v0.10.0 h1:X2//UzNDwYmtCLn7To6G58Wr6f5ahEAQgKNzv9Y951M=
 golang.org/x/net v0.10.0/go.mod h1:0qNGK6F8kojg2nk9dLZ2mShWaEBan6FAoqfSigmmuDg=
 golang.org/x/sys v0.0.0-20220704084225-05e143d24a9e/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
-golang.org/x/sys v0.0.0-20220811171246-fbc7d0a398ab/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
 golang.org/x/sys v0.6.0/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
 golang.org/x/sys v0.15.0 h1:h48lPFYpsTvQJZF4EKyI4aLHaev3CxivZmv7yZig9pc=
 golang.org/x/sys v0.15.0/go.mod h1:/VUhepiaJMQUp4+oa/7Zr1D23ma6VTLIYjOOTFZPUcA=